For another, and along those same lines, if you just ask me the question via email, you'll miss everyone elses responses. Other people might know a lot more than I do, and in fact, that's much more likely to be  the case than not.

Lastly, you deny others the chance to get their same question answered. If you ask first, and they search for that question, they'll have their answer right away, not after asking and waiting for people to post the answer.