Simple switch wiring
If you are correct about where power is coming in at (a bit more likely than not, but you'll need to test it or open up the box the middle cable goes to if you want to be sure), then the black wire in the middle cable carries power onward to some other thing that is not switched (like another set of light switches, or some receptacle or outlet somewhere, maybe a smoke detector?).  If power is indeed coming in on the black wire in the middle cable (somewhat less likely), then the left cable is going off to another non-switched thing.