The idea is to get things as close to ideal as possible. If you send a signal over a long distance, trim the impedances as much as possible to get maximun efficiency then you can reduce the generating power to minimum to get reliable reception.
To avoid distortion caused by power reflections and also to maintain a flat frequency response across the frequency band of the signals being transferred from source to load.
At pure matching condition, no reflecting power appears between load and source. This way, close to 100% signal strength applies to the load without loss.
impedance matching when done between supply and load it is done to maximize power transfer, when done on cables and buses it is done to prevent or minimize signal reflections, a low impedance mismatch reflects an inverted signal, a high impedance mismatch reflects an in phase signal.
To avoid reflections that also cause signal distortion. It does help with efficiency, but that is secondary.
