@RBEmerson described a good use case for intersection of gradient and auto-mask.
I think the feature request needs to be broadened to any combination of any selection: add, subtract, intersect.
The problem will be that the mask is no standalone entity yet in the UI of PL: it is always coupled to a local adjustment. So in the end, in the local adjustment layer, we may want to see masks as sub layers of a local adjustment layer with the ability to add a mask layer (e. g. control points) and set a blend mode for masks such as subtraction from an underlying gradient mask.
Or we do Photoshop/Affinity/GIMP/… for such pixel editing as they have all of this built in already and keep PL as the best RAW developer. But then people like to do everything in one tool and are unhappy with the transfer between different software packages and the nearly inevitable issue of destructive editing.