expensive but worthwhile!
It is a way of raise or lowering the engine by flicking a switch at the console. With a reasonably big engine tilting it up (e.g. for launcing, beaching, shallow water etc) is hard work so this takes the effort out. Then there is trim - fine ajustments of the transom-engine angle which allow you to alter the engine trim and therefor the trim of the boat (to raise or lower the bow). This can be used to help get the boat on the plane or to help grip in sharp fast turns.
The bigger the engine the more important it becomes. The more likely you are to do things which alter the boat trim (e.g. differnet loads, weather conditions, or towing skiers) also make it more important. I guess where/how you keep the boat also influences the need - keep it in a marina might be less important than launching from a slip.