>>64393970
I think it's just what the combine does. Takes over a territory, and combines what's in it with itself.
Humans are just free to use, walking bio-robots that you can recycle into infantry, engineers, and an occupation force while you wait for the free-minded humans to finish making your teleporter as, presumably, anything that's been "combined" with them loses the cognitive ability to create teleportation tech.
The brutal efficiency of the combine is what makes them so creepy and uncanny imo. It's more efficient to recycle us than to use infantry from other worlds/universes.