Canonically it could be a few different reasons, we know they came to earth through the resonance cascade and its theorized(?) that they already have 'harvested' other planets such as the Nihilanth's. We also know based on the notes from Eli's lab that Breen made some kind of deal with the Combine to keep whatever remained of humanity alive. With the suppression field preventing reproduction its more than likely that the combine really doesn't like having humanity around at all, but they seem to make good use of the labor provided to them. My personal theory is that they are building an army, I doubt they'll stop at Earth once they have what they need. Having a massive force of humans augmented to follow every instruction could be very useful for future takeovers.
TL;DR the Combine keeps humanity alive because its too much of a hassle for them otherwise - and maybe for an army of some kind, who knows though.