Multicasting primarily (or exclusively) refers to OTA digital broadcasts. For example, several of the digital stations in Dallas Multicast. They take their allocated bandwidth, and split it into 2 or 4 separate channels that they can broadcast concurrently on.
Unfortunately, it often means comprimising the bit rate of HD broadcasts, as bandwidth is being used for the secondary (or more) channels.
Some OTA channels broadcast weather maps, some broadcasts news channels, some broadcast infomercials, and a few even broadcast "The Tube" on their multicast channels.