Why do people ask questions and answer their own questions?
Too often people will answer the question they just asked to promote or support their own point of view. They may also want to show off a bit. Either way, if the question is phrased well and the answer supplied is "correct" as posted, then WikiAnswers is better for it. Anyone who shows up and asks that question and finds it already asked and answered has benefited from this informational resource.
A good reason, though, for doing this is to supply information in a Q&A style. If you know something, and think it may be a question people want to know about, you can ask a question and supply the initial answer. Just remember that WikiAnswers is a wiki, which means your answer can and will be "edited mercilessly" by the Community at large. I personally think that it is really weird!