There are too many reasons to even start a list
But for the sake of BREVITY lets pick just one for example which is the most common reason I use
Local adjustment of whatever......could be color, tone, exposure etc etc - local adjustment meaning a specific area only
In other words, when making an adjustment to a portion of the image and not the whole thing.
Sure you understand the mask, where you can apply specifically where you want or dont want it
The layer itself, if the adjustment is on a layer, you can remove or hid it later in the process if needed, even adjust it further.
Just one of tons of examples