Not all changes are caught, so...
I am a pretty good typist and, therefore, confident that what I type is correct. However, my confidence faults at the fact that I am not a "look at the screen as you type" person. Sure I make some typos here and there, but that is what the spell-check is for, allowing me to choose the correct spelling or word for what I meant. Having Word choose my words is taking the risk of not catching it doing so, thus giving a different meaning to what I am about to show or send to someone else, for the changes can often be such that even the grammar check does not flag either. This function works great when you want to replace a few letters with long phrases or sentences, like in a law office where much is repetitive and titles are long. Allowing Word to choose words for you is like using the online translators for a full document and believing that what they say is 100% correct.
Are you positive that you catch every instance? Doesn't it tire you to be ever so vigilant of POSSIBLE automatic changes?
Cheers.