There are situations when it is useful to scale the dynamic range of a signal down to a few coarse sub ranges. As an example you can think of an indicator that does not show the exact speed of a motor, but only a few values like Off, Slow, Medium, Fast and Too Fast. Such signals can easily be obtained by averaging (i.e. low-pass filtering) amplitudes, frequencies or pulsewidths of signals produced somewhere in the system that you want to keep an eye on.