Depends exactly what you mean by it, but there do seem to be beneficial projects that are best organized as for-profit companies. For example, I doubt a nonprofit would have done as good a job at search or email as Google has.

I wrote about this phenomenon here: http://paulgraham.com/good.html

Yes, though there's a lot to understand to be able to efficiently save the world; Every little bit and effort helps, though.

Contradictory goals? Saving the world generally means selfless work for the common good. Startup generally means creating a new business with the goal of making yourself rich. (I know that will piss a lot of people off.)

But if your goal of saving the world works out, you'll get rich on the speaking fees.