Someone who is a very good speller benefits from a spell checker, because he or she isn't necessarily a perfect typist. Even if you make one typo in 20,000 words, and the spell checker finds it, there is value in that.
Grammer checking is just extended spell checking. You could have great style, but gaffe something; use some combination of words because you were tired or distracted, or you made some bad edit, half of which was scrolled off the screen or whatever.
You could ignore most of the tips, but once in a while it could be that the suggestion you get from the software is exactly the words you wanted to write, but somehow fumbled. That's clearly valuable, and in a way that isn't dictating style to you.
Sometimes spelling errors cause grammar errors. So unless you're a perfect speller and typist, a grammar checking tool might be the one which actually catches your typo. This is because a typo can produce a dictionary word, or words.