In the computer science academic sphere, the conferences at which one is a reviewer is among the top items on a CV. It implies that you are such a pillar and expert in that community that they would ask you to referee the merits of incoming academic work. Usually to be invited as a reviewer at a top venue, you should have a history of successfully having your work accepted to that venue. That's very difficult as acceptance rates for top CS venues are generally around 20%, and even experienced submitters often have papers rejected. My paper was actually recently rejected from Oakland (a top CS security venue).
Now things become clear! I always thought as an academic you should just read and evaluate those papers you are interested in. Which for me is really a minuscule proportion of all papers submitted. Maybe academia as a business isn't such a good idea.
Being a good figure in a conference planning is a sought honor. Reviewing is the grunt work you have to do in order to get that honor I guess, that's the thought process.