User blog comment:Subject 9/Reasons/@comment-4355416-20110903163902

Think of the galaxy as a big fruit tree. What do you do when the fruits in that tree become ripe enough? You harvest them. They've perfected ways of controlling and converting organics for one reason: they need organics in order to survive. What type of sustenance they're harvested for is uncertain for the moment but we've already seen that they can make Reapers from people. There's no telling what else they use us for.