my understanding .... they are a great source of nitrogen (up to 2% or so), they do other good things as well like stimulate soil biology
the only down fall in my understanding is they dont have much P or K or things like calcium which you need a bit of, and the type of nitrogen the coffee
releases is ammonia based which decreases soil ph over time, as opposed to urea based nitrogen which is neutral or nitrate based which increases ph - so they
are best used in compost piles at the start of the soil making process whereby the soil bacteria can break down the ammonia compounds ... bla bla bla