The problem: streaming to 1000 viewers
If a video server sends a unicast stream to 1,000 viewers, it must send 1,000 separate copies of the same data — one to each viewer. At 4 Mbps per stream, that's 4 Gbps from the server.
Multicast lets the server send one stream (4 Mbps). Routers replicate the stream only at points where paths diverge. Viewers subscribe to a group address (239.x.x.x) and receive the stream automatically.
The network does the replication, not the server.