In this future flashback scene from the original Terminator movie, an infiltration unit gets inside the place where humans hide.
Why have an infiltration terminator at all?
Skynet could just get the location of the hiding place and toss in a few poison gas grenades. Or it could pull back its units and bomb the place from the sky.
This is kind of related to the question about why Skynet would want prisoners, but this is more focused on getting inside human hideouts rather than just rounding up prisoners.