I have started answering on Stack Overflow two months ago, and I'm giving up a lot of my free time.
What benefit I will get on a personal and professional level by answering on Stack Overflow?
Does it at least improve my knowledge?
You'll get an addiction to the green badge (+x reputation) on the achievement inbox in the top bar.
No, seriously, I've learnt countless things
Many things, most of which are dependent on your values as a person and if you see the benefit in them.