The point is to strengthen and expand the reach of a Power you pledge allegiance to. This will (hopefully) result in exciting roleplaying opportunities for players and devs, and emergent gameplay for those who don't care about RP.
You get rewards in the form of a weekly credit allowance with some added perks depending on the Power, and based on how much you contributed toward Preparation, Expansion, Control and Undermining in the past four weeks. What these four actions mean in practice is in the official PP manual and guide I linked to earlier.
Thanks but I was wondering about what powerplay is for, not the reputation system.