Hello, and welcome on Arturia forum.
You're right, for your setup, the only benefits to have the 8Pre connected to the computer using USB is to configure it through the AFCC.
It's not mandatory at all especially because you can access the ADAT channels gains/pan/mute/solo directly in the AF Studio mixer if you need to, and the 8Pre Inputs all have physical controls on the hardware device.
So if you use the 8Pre as an ADAT expander, and you confident that you will not need to often change the 8Pre settings, feel free to unplug the USB from it