It’s not all altruism. Answering interesting questions is a fabulous way of improving your own knowledge of a particular topic. It’s not until you start to explain something to someone else that you realise all the gaps in your own understanding. Many times, I would see a Stack Exchange and think “that’s a quick and/or easy question to answer” – only to discover there are more subtleties and possible edge cases to consider once you really start thinking about it. Before you know it you’ve spent over an hour trying to answer the question as comprehensively as possible.
> Answering questions also can make you feel smart.
Not so much. Though, when I read answers that I wrote years ago, I think my past self was a lot smarter than my present self.
I’d also view Wikipedia and Stack Exchange with their GFDL and Creative Commons licenses as being the textual equivalents of the FOSS world. Both are projects that I’ve benefited a lot from so it’s nice to be able to give something back.