It's a cheap 4-channel headphone amp.
Route your signal in, find your favourite pair of cans, dial in the volume, hand another pair to your co-conspirator.
I also use it to A/B different monitoring headphones whose characteristics I know well, in a feeble attempt to combat ear fatigue (but really, to catch what pair 1 missed, with pair 2).
It's a problem-solver for me; as it solves the problem, that's five stars.