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
- while searching for the answer to another person's question.
- after reading a 'competing' answer with a much better approach than I thought of
- by browsing the Hot Network Questions to 'relax' after working hard on an answer
- while writing a detailed explanation about some code I posted as an answer. Writing good code is one thing; being able to explain the principles behind it is another, which can help in transfering the code you write at work to your colleagues.