why multicast?

because there are groups of U.S. Marines as well as groups of U.S. Army soldiers for whom the Air Force needs locations. Multicast groups can be formed of groups of smaller groups. GPS data can be shared by multicast. All group members are provided data for that multicast.

Multicast groups can be formed according to need and dissolved when usefulness is finished.

If perhaps a group of teenagers wished to share their real-time GPS upon a shared map, multicast provides the solution.