I had my first close encounter with a horned tree hopper the other day. This small sap sucking homopteran is associated with young oak trees and does not seem to be very common.
The purpose of the 'horns', if there is one, would seem to be to frighten would be predators off as they do not seem to be a display item.
The pronotum also runs backwards over the top of the body like some alien's armour and might, I suppose, have some protective function.
I find these complicated forms quite difficult to grasp in evolutionary terms. If such excrescences are of value, why do so few creatures develop them. Most homopterans, for example, have neither horns nor a backplate, nor do they appear to be on the way to developing them.