PIN diode square wave AM modulation

What is the aim of PIN diode square wave AM modulation in Microwave measurements ?

A pin diode used as hard squarewave AM modulation would not be good for any comm link due to wide spectrum.

It would be useful in measurement of automatic gain control response in a receiver. It can measure the attack, decay, and any instability in the receivers gain control circuitry.

I have used squarewave AM modulation on an HP RF test generator, which had about 200 kHz modulation bandwidth, to measure AGC response in a narrow band receiver.

What is the theory behind measurement of RF Micro Wave on a narrow band device, using PIN modulator ?