I think you're mixing two things:
In analog AM transmission they're related since audio signal modulates the amplitude of the RF carrier. So if you want to use the full power of your transmitter, but not exceed it, then you need automatic gain control to set the analog signal peak level to where you want it.
But in analog FM and digital transmission they are not, the transmitted RF power is pretty much constant. In FM the frequency changes but the amplitude stays constant, and in digital transmission it depends on the modulation scheme used, but most will try to maximize signal to noise ratio by keeping RF transmitted power high and constant on average, for example by modulating phase. If it also uses amplitude modulation, the signal is scrambled to keep average power constant and not dependent on data being transmitted, which gives maximum SNR.
In both latter cases you still need to maximize the use of available audio signal amplitude range without clipping, so you'd still use some form of AGC, but it would rather be called "limiter/compressor" or something like that. It can be implemented as an analog effects box in your rack, or as software.
For example compression reduces the dynamic range, to make soft parts of the music a bit louder, since the signal to noise ratio of AM/FM is not that good and people listen in their car where there is a lot of noise anyway.