If we have spell check and use it, why do we have to worry? If it works, why are there so many errors on blogs, in emails, even in the newspaper?
There are so many errors the spell checker misses that they’re hard to enumerate. It confuses “it’s” with “its,” it doesn’t like fragmented sentences, even if the author does. It will find a word unwittingly repeated, but doesn’t alert you to overuse of a word or term.