Reading about Voyager in The oldest computer (not) on Earth I saw the image below, where is shown a science instrument calibration panel and shunt radiator.
According to the November 1980 NASA News Release Voyager Backgrounder:
A .36-square-m (4-square-ft.) shunt radiator/science calibration target faces outward from the propulsion module truss adapter toward the scan platform. The dual-purpose structure is a flat sandwich of two aluminum radiating surfaces lining a honeycomb core. Through power collectors and emitter resistors between the plates, any amount of the electrical power from the radioisotope thermoelectric generators can be radiated to space as heat. The outer surface also serves as a photometric calibration target for the remote-sensing science instruments on the scan platform.
Why would it be necessary to build a system to radiate unused electrical power as heat? According to this table each of Voyager's three MHW-RTGs started with 2400 W of thermal power producing only 160W of electrical power. I assume that using or shunting the 160W of electrical power results in 160W less radiated power, but why was providing a maximum 7% drop in radiated power worth adding the weight and complexity of this electrical and radiation system?