Input/Output Preamp Controls

[NJS]

Hey all,

Ive been doing some work on a Vintech Dual 72 for awhile now but I never understood the purpose of having two separate controls for input and output gain.Obviously many high end preamps have these controls but why? Also, how do they effect the tone of sound Im hearing? I know that depending on how I set the controls lots of natural compression can be created, why does this happen?

Thanks for any answers!