Sure, here's a simple use case. Imagine making a Keyboard patch and wanting a percussive attack. I want to preserve all four ADSR envelopes for interesting purposes, such as filter control, wavetable position, etc. So, if I'm using SubOsc2 as white noise, I can simply use the FG as an AD envelope to control the SubOsc2 Volume knob. And I could also have it applied as an additional envelope for the filter, etc.
EDIT: Scratched out a quick example patch as attachment. Not an actual good patch, just a mod assignment example.