Dear Shezza,
I used to have a wonderful Hammond, and the expression pedal is a basic component of playing an Hammond organ. The fact that usually it is called "expression" and not "volume", tells the difference. It is the same with other instrument, like, for instance, saxophones and many others. If you keep the volume static, you get a not natural sounding. Expression (volume) pedal instead helps you to create a more natural sound.
In Genos it is very easy to connect a volume pedal, and assign the pedal action to the parts that you prefer, according to the instrument (voice) that you want to play, and accordingly with your personal taste.
In my case I have a Yamaha FC7 pedal, controlling all keyboard parts (Lower, Right1-2-3), on any voices combinations, but not on style parts.
Besides organ voices, it is nice to use the volume (expression) pedal with pads, choirs. And the pedal action is very effective on other instruments as all brass, synth, ecc.
I don't use the pedal with decay voices, as piano and guitars, where I prefer to control the sound with velocity.
It is a matter of personal taste. But organs do require the expression pedal.
Ciao
Angelo