Teach, to learn. There is no better way to learn than to teach others, at the same time teaching yourself. Teach, and, in the long run, it will be a test of how well you know what you know.
Source: (dead link – http://www.quora.com/Why-should-I-provide-my-knowledge-to-Quora-for-free) Henry Shady’s answer to Why should I provide my knowledge to Quora for free? – Quora