Episode #45 of the Stack Exchange podcast featured Eric Lippert, who got some great advice from his first manager at Microsoft:
I want you to be a recognized industry expert on something. ... But don't pick something that's too big. ... Find a source of questions, answer every question that you know the answer to that is in that domain, and if there is a question that you not know the answer to, make it your business to find out.
This is why Eric Lippert answers so many Stack Overflow questions.
This is why everyone should be answering more questions and sharing their expertise. It makes you better at what you do.