A knol by Johnson C. Philip offers some legitimate reasons why you should contribute knols.

According to Mr. Johnson, here are the five reasons why you should write knols:

1. To help, motivate, and guide others
2. To better use free time by making writing knols a very special hobby.
3. To establish yourself as a writer, which will later be a mean to earn some extra income.
4. To sharpen your way of thinking.
5. To ‘leave your mark’ in this new media.

I would like to add number six: To promote and educate visitors about your websites and causes through knols you write, knowing that Google Knol will be bigger, more visited and more ‘social’ than Wikipedia.