Purpose : combining mask tools provides finer control over where and by how much. Mask tools include: gradient, control points, brush, and automatic mask. The eraser would still be applicable to mask tools.
The user interface would allow the user to create an effective mask, by associating masks created using different tools.
The effective mask is associated to an equalizer control panel.
Example : correct sky using a gradient, while preserving mountains and trees. The desired mask is the product (intersection) of a gradient and the identification of the image area defined by other means, using for example control points. The resulting mask applies a local adjustment modulated by a gradient to parts of an image.
This has sort of come up before in discussions regarding luminosity and colour range masking. Not sure that the control point, which masks on a particular colour or tone, would be sufficient with a gradient or brush.
@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.
Lr/ACR and Capture One have decent implementations of colour range and luminosity masking. C1 has traditional layers whilst Lr/ACR does not. It is also possible to add and subtract from their selections with the adjustment brush which gives even more control. I have found I can get quite good selections in PL but not as easily. The idea of keeping PL as a base raw editor is a thing of the past surely - it is much more than that already - and good that it is its future is keeping up with the others as they too are decent raw editors.