One of Multicasts main benefits is bandwidth savings. without going into the differences between sparse-mode/dense-mode here is a basic example
Server1 needs to send the same data/packets to 3 hosts across 3 routers.
Instead of creating 3 separate unicast packets to each host destination, Server1 sends 1 packet to 1 multicast group address,
R3 would recieve the multicast packet, duplicate, & send to the hosts.
1 packet-----------------------------> Host1
Server -----------> R1 --------> R2 --------> R3 ----------------------------> host 2
3 packets to all 3 hosts ----> host 3
In a case where you have hundreds of hosts needing to receive traffic for a multicast group address, The bandwidth savings are tremendous especially over WAN links
