Chapotraphouse

Their interest in Earth seems to stem from how Earth's sciensts created teleportation within the same universe, which is something the combine was never able to nail despite their resources. That meant humans were a bit of a liability, but they didn't seem to think it would take much to control and manage the population. Or who even knows, the combine might even have a hard time making plans since they're so widespread and may not see time the same way as humans.