the relay also reverses the polarity of the switched signal. since the horn itself is grounded by its mounting bracket it requires the positive voltage to be switched to activate it. at the same time, it is much more convenient and safer to have the button on the wheel work by grounding out the ring behind it in order to avoid running a hot wire up to that moving component. so the relay is a easy component to add for current capacity as mentioned by Gord&Fran and also serve as a way to reverse the switching polarity. the relay is activated by grounding then provides the positive signal to the horn...
