+1 for Eric and Barry.

Though we can list benefits like better writing skills, networking with similar people, global audience for our content, it is very hard to make someone a returning contributor based on these. Global / Indian education program is a good example. Even after intense and direct outreach, the conversion rate there is less.

I have done 10+ physical outreaches and the conversion rate is very very less. Outreaches are very much a branding exercise to increase awareness about Wikipedia. Not everyone can and will become a poet or a musician or an environmentalist or a human rights activist even if we do 100s of workshops. The same holds true for Wikipedia.

Wikipedia is a social cause in which the right contributor finds joy instantly. It is similar to the joy a kid feels at seeing a ball. You need not educate a kid about the ball but you just have to introduce it. People asking "what's in it for me in Wikipedia" are similar to people who ask "what's in it for playing". Almost 90% of very active Tamil Wikipedia editors jumped at the project once such a thing existed.

So, we should not worry about the people who ask "Why". Just spread the message to as many people by mass media ( Even a TV ad campaign will be worth the spending ) and focus on the people who ask "How". There are many such people who can relate with the cause of Wikipedia but they just don't know that it exists and they can contribute.