I've recently calibrated my MBP's display and my LG 5K 27" display at home.
When I'm on the go, at school for example, I would like to be able to raise or lower my screen brightness without screwing up the calibration. It was kind of a PITA to get both displays to perfectly match in brightness.
I'd like to be able to sort of bookmark my current brightness so that I can later restore it.