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 considering the print could satisfy a different, even distinct goal.