“...that they would otherwise not have the power to transmit.”
Interesting.
(Going from memory here):
The Martian Rovers were really pretty small. They needed to be large enough to be stable while moving and with equipment extended, but small enough to be landed softly enough not to break. There solar panel arrays are not large either.
It was determined to not equip the rovers with transmitters that could routinely transmit to Eafth, but rather they would send compressed messages to MRO each time it passed over them in its orbit. MRO would then relay the compressed message to Earth with its own powerful transmitter. MRO has been very frugal with its propellant and has plenty energy remaining to change orbit as needed and to transmit messages almost indefinitely (within the reasonable lifetimes of the surface rovers.) MRO was launched about 20 years ago.
This stuff is pretty cool, really.