Questions have costs?!!?

To prevent the site being flooded with nonsense. It doesn't. You agreed to the Terms of Service when you joined. You did not have to waste points asking this question. Instead read the topic "All About Answers" located on the left of your screen on YA home page.

To encourage users to answer.

To thank the Yahoo! Answers community for great answers, we give points for various actions. Answers points help you build your reputation on Yahoo! Answers and show the world what you have to offer. You need points to ask a question but you can quickly earn the points back when you resolve your question and by giving answers that are picked as the best answer.

The points table linked below summarizes how many points you get for each of the different actions you take. The points allow the community to recognize how active and helpful you've been, and they give you another excuse to brag to your friends.

http://answers.yahoo.com/info/scoring_system

The levels mark your activity and show what Answers usage privileges you earned. The more points you accumulate, the higher your level, and the more privileges you get. We’re planning some exciting things for Yahoo! Answerers, so stay tuned!

My guess would be to encourage you to answer other folks' questions instead of just asking a bunch.

i know

what thy do.