"Why do I need an editor? Can't I just use Grammarly?" is a common question from authors. The short answer is because a grammar checker is only as good as the person using it. Without the skills required of an editor, using a grammar checker can cause more harm than improvement in a manuscript.
First, there are things a professional editor can do that grammar checkers aren't equipped for. These include:
Structural edits: While grammar checkers can give suggestions to improve structure at the sentence level, it can't help you tackle big-picture concerns such as character development, setting, plot and overall structure.
Story consistency: While programs like PerfectIt will flag that you've spelt theatre both theatre and theater it won't flag that the colour of your protagonist's eyes keep changing. It's not going to pick up on the fact that Jimmy used to be Johnny and that Sally has disappeared from the narrative without explanation. It won't flag that 70% of respondents said they mistrusted AI on page 3, but on page 10 the number is 85%. These are all things that require the reader to understand the piece as a whole and reference (with understanding) content potentially dozens of pages apart. Software can't do this.
Stylistic/formatting choices: This is especially true in fiction. There are multiple "correct" ways to spell a word (colour vs color) and sometimes a word is misspelled on purpose. While consistency matters, there are valid reasons to use multiple spellings in a single piece (as I did with colour/color). Capitalization can be used to signify important story elements. Italics and boldface can be used to signal specific story modes or types of text. Hyphenation and comma usage can follow rules specific to the text. An editor will not only maintain the rules you have set out, but they can help you shape these rules to create the best reader experience.
Explanations and guidance: An editor can give detailed feedback on why a change should be made. This is especially important when dealing with stylistic or line editing which is about more than just following the rules of written English. While grammar checkers usually give some explanation of the changes they recommend, these are usually generic and final. An editor provides feedback that is specific and in a dialogue. Authors can ask an editor questions and discuss options.
Distance and a subjective, objective eye: Most importantly, the editor provides a trained eye that has some distance from the material. Even if you are an excellent writer, it is a well-known fact that it is difficult to edit your own work, regardless of your training. Using a grammar checker is still editing your own work. This means that more things will slip through since we always overlook things in our own work.
Help with rewriting: There are some things (like overly long, complex or repetitive sentences) that grammar checkers will flag, but their guidance stops there. An editor can help with tricky rewording.
Knowing there is more to good writing than correctness: This is where the human touch truly cannot be matched by software. Grammar checkers are concerned with following the rules set out by their programmers, but just because something is correct does not mean that it is well written or that it works for the story being told.
Sometimes, a grammar checker can create more work, especially with manuscripts that have a lot of deviations from "standard" grammar and spelling or manuscripts that use multiple languages. While some grammar checkers can work in multiple languages, they can only work in one at a time. If you are writing a fantasy novel with lots of invented names or a science fiction novel with lots of technical jargon, each use will be flagged which means wading through hundreds of non-errors to find the actual errors. If you're working with a conlang, this intensifies the problem. In these cases, there is a tendency to get into a rhythm of rejecting all the suggestions, meaning errors are often missed.
It is possible to program the software to recognize individual conlang words and character names, but this can be time-consuming and is a blunt instrument. The software will accept every use of these words as correct since it has no context for what they mean. If you are working with invented grammar and/or syntax, the software is useless.
This issue of grammar and syntax doesn't only apply to conlangs. It also applies to many things written in non-standard English. While the better grammar checkers might leave common spellings such as ain't alone, they are likely to flag legitimate dialectical constructions such as "Where you to?" "He say" "To move forward in a good way" or "She be working" as incorrect. All of these terms are in common usage somewhere in the English-speaking world and as a result, are correct usage, especially within dialogue.
The biggest thing to remember about grammar checkers is that humans built them and therefore have human biases built into them. They have been programmed with a specific set of rules that the programmers have deemed correct. While human editors also come with their own biases, they also come with three advantages that software does not.
A good editor is aware of valid constructions beyond their preferences and knows when to query instead of changing. Because human "programming" is more flexible softwares', a human editor can operate using multiple "rules" of language at a time. A human can make a judgment call to bend or break the rules where appropriate. This ability is especially important in fiction that uses different dialects or rules of English to demonstrate character, time and place.
A human editor comes with more transparency than a grammar checker. By speaking to them, you can get a sense of what their biases are and pick someone who gets your work. You can also ask them to explain their choices and this explanation will give you an idea of how they work. Software is a black box. There is no way to know how it arrived at its conclusions or even the full set of rules it is operating under without performing many sample edits on many different kinds of text. Software updates can change these parameters at any time without warning meaning you can never be fully sure what rules they are operating with.
An editor can look at the piece as a whole. Software is limited in the scope it is examining and can only look for the specifics it has been asked to look for. While editors focus their scope, they can also let you know if you've hired them for a proofread but what you really need is a structural edit. The grammar checker will only do what is asked and you'll miss important advice that goes beyond its very specific lane.
It is also worth considering that extensive use of grammar checkers makes your work more likely to be flagged as AI. As AI becomes more prevalent, it is likely that some publishers and agents will start to use software to weed out AI-written submissions. In the future, it is possible that the extensive use of grammar checkers could reduce the chances of finding an agent or being traditionally published.
This is not to say there isn't a place for grammar checkers in an editorial workflow. But remember: a grammar checker is only as good as the person using it. To be an effective tool, there needs to be a person who understands the rules of language, the style of the writing and the intent of the text making judgment calls as to what suggestions are valid and which aren't. Otherwise, meanings can change and the authorial voice can be lost. This is one of the biggest pitfalls of grammar checkers; they seek to make everything read the same.
Final Thoughts on Grammar Checkers
No two editors will provide the same feedback on a work, and the same is true of grammar checkers. Just like editors, grammar checkers have preferences. While software has its uses, it can only operate under the rules of its programming. Humans are flexible and creative in a way that machines aren't. If you are experimenting with language, an editor can tell you if it's working or not, and if not, can help you find something that will. A grammar checker will just tell you it's wrong and cite the rule book it has been programmed with. While there is some room for flexibility in software, it is nothing compared to the flexibility of the human mind.
In general, grammar checkers are always trying to reduce the word count and make things more succinct and uniform. But the goal isn't always to convey information in the most efficient manner, especially in fiction. Sometimes the rhythm or the journey is the point. Sometimes the goal is to get inside the character's head, so the words on the page mirror their state of mind. Sometimes the goal is to make the reader work to understand.
Unless the end goal is for a text to be read by robots, grammar checkers are not enough to get a manuscript to its best possible point. Because they require rules as part of their programming, they can't address the portions of a manuscript that are more fluid like character traits and story elements. An editor or reader assesses more than how correct the writing is. A person can appreciate the effect of an intentionally broken rule. Software can only tell you that you've broken the rule. Correctness matters, but that is only a part of what makes good writing. The other part is intangible and the only way to assess that is with human feedback.