This is critically important in redundant multicast topologies. Because the same multicast packet could reach the same router via multiple interfaces, RPF checking is integral in the decision to forward packets or not. If the router forwarded all packets that come in interface A to interface B and it also forwarded all packets coming in interface B to interface A and both interfaces receive the same packet, this will create a routing loop where packets will be forwarded in both directions until their IP TTLs expire. Routing loops are best avoided as they unnecessarily consume network resources.