Reasons for masking:
Primarily, two: adjust contrast, or, adjust hue. These are often used as corrections, working toward a improvement in reproduction due to some deficiency– the actual will not be as good as theory requires.
I learned masking as part of the steps of making dye transfer prints. Masks and seps were so commonly done that it didn’t seem difficult or unusual to make them. As I gained skill, I moved from reproduction of the original to realizing that the print could satisfy different, even distinct goal. This has translated to the digital work as moving imaging from correcting color to making color for translation. Instead of taking color away, I can add color.
Within my dye transfer work, I call this hue bending.