The three pillars of academic life are research, teaching and service. (We don't consider endless faculty meetings to be a "pillar"; they're more like a pillory.) One aspect of service is performing (uncompensated) reviews for journals. I've gotten my share of requests, and usually accept the assignment. In fact, I'm working on one now. Sometimes, though, I just have to turn one down, as the following redacted exchange demonstrates.
Reviewer response form:
There's more to the story, but the rest will cost you a beer at some conference.