Multicast moves the role of packet replication for multiple recievers from the source host to the routers that make up the network between the source and all the clients. When deployed nativly (MBGP + PIM)this insures that each network link carries only one copy of the packets, rather than one copy for each reciever on the other side.
The result of this is that applications which stream data such as video, audio, stock quotes, usenet news, weather data etc, can scale to serve thousands or millions of users with modest resources.
By way of example, lets say you want to serve a live rock concert to 50,000 fans over the internet, if you did this with a unicast 56Kb/s real-video stream it would require that you provision bandwidth between you and your customers equal to 50,000 * 56Kb/s or 2800000Kb/s, 2800Mb/s or 2.8Gb/s just slightly (400Mb/s is more than slight I guess) more than an oc-48, in addition, assuming you could serve 500 streams per box, which is somewhat optimistic, it would require more than 100 pc servers. Using multicast, your source needs one pc encoder, and 1 64Kb/s isdn B channel. Each router in the path will recieve exactly one stream and replicate it once, onto each interface which has downstream recievers. Put another way instead of sending postage stamp sized video you could sent a 1.5Mb/s mpeg-1 video stream people could watch on their bigscreen tv's and the cost would be only incremental to you and your upstream provider and your customers providers