I was just curious if this device serves any function besides tearing your boarding pass and giving you the stub-end (i.e. does it read anything from the bp?)

I had an interesting experience a few weeks ago. My wife and I were among the first to board and sat down in our seats (3a+b). About 10 minutes later, another couple tells us that we are in their seats; we compare stubs - they have the same seat numbers. The attendant walks them back to the gate with all four stubs. It turns out that their stubs were on a different flight.

You would think that having every ticket scanned at the gate would fix this issue - either the machine would complain that "the seat has already boarded" or that "you have the wrong flight number".