Wikipedia:Why on Earth would I want to contribute to a wiki

1. It's fun.
2. It's easy.
3. It's educational.
4. It's social.
5. You are part of an exciting development making use of the open source idea.
6. You can correct other people on the spot without asking their permission!
7. It's exciting. Wikipedia has created literally thousands of articles in the mere year and a half since its inception--more than 2,000 per month, in fact.
8. It's interesting to share your knowledge with other people, particularly knowing that others can instantly edit or react to what you've written.
9. To contribute is to gain.

See also why Wikipedia is so great.