It is an HF radio for longer distances. I don't think it is absolutely required, but to give position reports to ATC, it would be nice to not need an airliner to relay the messages.

Jeff Hersom N3740J '67 150G "Gremlin"
Hangar W-6, Helena Regional Airport
Places I have landed Gremlin:
[Linked Image from visitedstatesmap.com][Linked Image]