One of the nice things about the D*TV service plan is that is covers service calls if you wish to relocate a receiver at the same address. Let's say you want to move a receiver from one room to another or to a different location in a room, requiring new cabling (i.e. they'll have to run two new wires from the dish to your relocated DVR because you wanted to swap the furniture with the television in the living room).
Without the plan, you pay the $80 service call to do this. If you sign up for the plan and then don't wait 30 days, it costs $20 for the call. If you sign up and wait 30 days for service, it's free.
As others have mentioned, the annual cost of the plan is about the cost of one service call. However, if nothing goes wrong with your equipment for x years, then you've paid x times the cost of a service call in premiums. I imagine that some people may omit the service plan, then wait till something goes wrong, then enroll and pay the $20 for the subsequent service call because they can't wait 30 days. This would save premiums and limit the out of pocket cost to $20.
I assume that relocating to and re-installing equipment at another address is covered whether or not you subscribe to the service plan. This is what they have been advertising.