There are many reasons to spend our limited free time in StackOverflow (e.g, the pure interest of helping others, reciprocate after some of your own questions have been answered,…), including the obvious one of building a reputation and bringing some people to your own site (if your site talks about topics you answered to in StackOverflow, you will naturally link to some of your own posts as part of the answers since sometimes you’ll honestly believe that your post is the best way to complement the answer).
What you may not be aware of is the huge impact of participating in StackOverflow can have on your number of visitors. For the modeling-languages.com portal, close to 40% of my referral traffic in the last 6 months (9000 visits) has come from StackOverflow. Compare this to all my efforts in promoting the site in twitter (2nd place, 13%) and others.
A great combination: help others and get visitors at the same time!