20W is a lot to pump into a piece of coax if it's not going to an antenna. Besides usually the practice is to simply put the signal handling equipment at the antenna, or down convert to a lower frequency like an LNB does.

I would imagine it's more for a local microwave link. Microwave antennas have huge gain due to their directionality so 20W can do a lot. And you are limited to the horizon anyway.