The reason the relay is needed is the trailer has four pins, one for the ground, one for the tail, one for the left brake/signal and one for the right brake/signal.
Problem is that the Xterra has an amber signal and a red brake, so they are on seperate wires. So the relay makes it that the signal can keep on working while the brake is pressed. It also makes sure you don't overload the signal relay.
It's also possible to buy a
universal power module with relay that you need to hard wire to the battery and into each of the vehicle wires for tail, brake and signal. But they are a pain in the ass and in the same price range as the vehicle specific kit, so why bother. I used a universal kit for my Civic and I would never go through that trouble again if I can get a the vehicle specific module.