Yeah, I always got the impression that the Combine were basically just going to use humanity up, just like how they're doing with the natural resources - extract whatever value they can, and leave behind a completely barren planet, kind of like Tyranids in a way.

Humans could be used as laborers (I'm assuming there must be some kind of industry running on Earth, someone's gotta make all those MP7s, although I don't know if there's any bits in the lore clarifying that, I feel like it'd be kind of weird if all the citizens were just sitting around doing nothing all day), or as bodies to "enhance" and turn into soldiers (just as how a bunch of their fancy tech, like the gunships and the striders, were originally actual aliens, which were subjugated and turned into half-machine tools of war). During Nova Prospekt, Breen threatens the troops with "off-world assignment", so it's possible Combine soldiers have already been used on other Combine planets (and since that's a threat, they probably weren't exactly some elite force, but cannon fodder sent in to die suppressing some rebellion).

They've blocked reproduction, so it seems like they're not even interested in breeding humans for some nefarious purpose (the way they presumably are for the species that serve as the basis for synths) - to them, we're just a resource that they might as well get some use out of, but not valuable enough to otherwise cultivate.

There is one specific thing that they want out of humanity, which is teleportation tech - that's why they capture Eli and Breen personally talks with him to try to convince him to join them. They want the secret to the tech, since their teleporters are fucking trash. But beyond the few surviving Black Mesa scientists who were involved with that, they probably don't consider any other humans particularly relevant.