Nothing. just a dead weight for me. I had one installed on my P2, but eventually removed it. P3 and P4 are so reliable that it's a waste IMO. I appreciate that some are specialized in going too far and use RTH as a flight mode, (where RTH is actually an emergency procedure), but if you fly reasonably and don't put yourself in the possibility of loosing it, you shouldn't have to look for your drone and locate it in the middle of nowhere. Hardware failures could happen of course, I heard of them, never seen any. Got 4 drones, P2 P3 P4 and S900, and I feel perfectly fine flying without a tracker. I always bring them back home safely, and luck has nothing to do with it.
Just my experience...