We obviously can’t assume to know and write every possible way a user might elect to work with the Control Ring, but here are a few possibilities to get your thought process working.
First off, the most obvious suggestion: consider using it for something you want or need frequent access to, especially if the need for changes comes up during shooting. A video user, for instance, might want to be able to quickly access his or her ISO setting, when recording with the camera in Manual exposure mode.
A few other considerations:
Exposure mode you’re working in A photographer or videographer using Manual exposure might prefer immediate access to the most important exposure elements (that is, shutter speed or aperture, or possibly ISO). On the other hand, a no-brainer for anyone using autoexposure modes might be to put Exposure Compensation at the dial.
The lighting you’re working in Especially if lighting is constantly changing, grabbing a handful of ISO with the Control Ring can make a huge difference in your imagery as scenes or lighting varies. Or, if you rely on Auto ISO to take care of those adjustments, consider using it for quick changes to another element of the exposure triangle — shutter speed or lens aperture.
E-TTL flash shooting Remember, the EOS R is fully compatible with Canon’s EX-series Speedlites, and E-TTL II flash. While flash exposure compensation unfortunately cannot be set at the Control Dial (the “Exposure Compensation” icon refers to ambient exposure adjustments only), a flash user managing available light with the camera in Manual mode could certainly speed up the process of working with different ambient light levels by assigning shutter speed to the Control Dial. Or, that same shooter working with E-TTL flash in Av mode could quickly make similar adjustments with the aforementioned Exposure Compensation applied to the Control Ring.