Spell Checking Out Loud vs Online
Sometimes people think that a free spell check function will do their proofreading for them, but you have to be your own spell checker to stay on the safe side. There are times when an automatic grammar check function will mistakenly fault your grammar, so you need to combine those functions with personal proofreading. You also need to check a thesaurus dictionary when you recall thesaurus words off the top of your head, in order to be sure that you used them correctly.
There are some grammar check devices that work incredibly well, and others that will destroy your grammar. So the answer is to never assign the task of proofreading to an automated free spell check device, because the human eye is still the best spell checker. The only automatic functions that are often smarter than writers are thesaurus dictionary websites, because there are so many thesaurus words in the English language that few people understand them all correctly.