Just because you know that this is possible and encouraged doesn't mean everyone does. New folks show up here every day, and maybe a few of them know something you don't - encouraging them to write about it benefits everyone.
Besides making this more obvious to folks who skimmed the FAQ, it also avoids the confusion inherent in seeing someone post a question, starting in on an answer, only to see them post the same thing half-way through. Naturally, other people can still post answers to questions you've self-answered - but by having your answer there from the start, they need not waste time duplicating your efforts, and can instead post only when they have something new to add, or think they can do a better job than you did.
More details on this can be found on Meta SO: What is this “answer your own question” jazz?