To watch porn and make beats and once. Really, just more site. I personally use it just to be able to see more of what's going on. Some people use it when rewireing to see two programs at once. For me, one big thing is when i use protools, i can look at the large mixer and the sequencer on seperate screens and usually get everything in i need to see. Instead of having to switch windows.
Honestly, i sold my mpc to buy two flatscreens, don't regret it one bit (lol, that's blasphmous). I was finding my mpc was collecting some dust, i was still using sometimes, but still, i like it that much. Highly recommend