Pruning in PIM, whether SM or DM, is to prevent multicast traffic from going where it has not been requested.
With unicast IP packets, traffic is destined to a single host across a routed network, but multicast is destined to a group address with no specific destination. That means that without controls, multicast traffic would go everywhere, even where it is not needed or wanted, and it is a small step to overwhelm a routed network with multiple multicast streams.
Multicast protocols have been designed to keep multicast packets going to only where the multicast packets are wanted. It starts with IGMP, where a host tells its multicast router that it wants to receive traffic destined to a particular multicast group. The multicast routing protocols will the direct packets destined to that group to the router. If there are no hosts requesting packets destined to the multicast group, the the packets can, and should be for the network health, pruned.