Unless you are using a proprietary, dedicated grammar checker, auto grammar check is far from perfect and will miss some subject-verb agreement errors, incorrect use of idioms, or choppy flow. Particularly in professional environments where you are being evaluated, this can tank an otherwise solid piece of written work. Even online in HN comments, people will poke fun at grammar, and (while I don't have objective evidence for this) I have noticed that posts with poor grammar or misspellings tend to have less engagement overall. In a perfect world, this wouldn't matter, but it's a huge driving factor for why people use LLMs to touch up their writing.