For many applications, it is necessary to drive many speakers with a single amplifier, giving greater coverage to multiple rooms or over a given area. Constant voltage systems (usually 100V line) employ a transformer in the amplifier to convert the output to high impedance and a corresponding transformer in each speaker to tap of the required power from the amplifier.
Theoretically, this allows a large number of speakers to be driven by one amplifier, so long as the overall power being tapped by the speakers is within the amplifier’s output capacity (usually 90% of the amplifier’s maximum rated output).