I read that when a multicast aware router without receivers received a multicast message, the router will send an upstream prune message. I don't understand what is the purpose of the prune message. I read it in DVMRP - RPF:Pruning
It is quite simple - if router has no receivers for such multicast traffic, it has to say upstream to stop send this traffic to it, because nobody behind router wants this traffic. It keeps network resourses. Just imagine if it is TV traffic, and upstream send to us 100 chanells, but our users want only 30 from this bunch. Why we need other 70? It can be gigabits.