ok i was a dj for 3 years.
u wear headphones to get the base line of 2 tracks to match b4 u mix the tracks into each other givin a clear un-noticeable mix, also used for gettin a gd clear sound play back of how the microphone is coming accross and u can test all audio going out for example doin a set with a band u can test the levels and balance the 2 out 2 each other