Why does an optical power meter have two heads

Optical power meters for low power lasers come as separate heads and readout units, with multiple heads compatible with each readout unit. The term "optical power meter" may sound generic, but in popular usage, it specifically implies a fiber optic power meter. Other general purpose light power measuring devices are usually called radiometers, photometers, laser power. Keysight optical power meter heads serve as the sensing front-end that converts optical signals into electrical output for measurement. [pdf]