Basically, the only incentives are to slightly improve your resume by showing you are a reviewer for reputable journals, and to get fee waivers for publishing your own work in said journal (in crappy journals, usually). But you if you review a lot, you may get selected to be an editor and then climb the ladder from there, to be editor in a better journal, or become editor-in-chief, all of which can be prestigious (and paid) positions.