If this kind of reward didn't exist most people would just ask, and never answer.
I think people would still answer questions even if there were no XP. Look at the Perl mailing lists. People asking there still get answers, and the responders don't get any "reward" except for feeling good about helping out.
Another interesting thing about Experience Points is that it let you know how good are your answers.
This isn't necessarily true, either. Plausible sounding answers often get upvoted even if they are wrong.