Electrical power is a precious commodity on Mars. The Rover requires power to operate. Without power, it cannot move, use its science instruments, or communicate with Earth. The main source of power comes from a Radioisotope Thermoelectric Generator (RTG) and a solar array, but ultimately the batteries inside the body keep the Rover alive.
The first power priority is to keep the Rover 'warm' enough to operate. If the Rover's components get too cold they may permanently fail, and power must be diverted away from operation of the vehicle and instruments to keep the Rover warm.
Learn more about the role of the Power Engineer in Mission Control.