I have always struggled understanding the use of masks in design tools like Photoshop, Illustrator, and Sketch. I recently had to work on translating some creative from Sketch into a responsive design. I realized that a mask in these tools is like a containing element in HTML with overflow hidden and the contents being absolutely positioned.
This is not something you would want to do a lot of as there is a lot of the image being loaded that not being shown. But, it is useful in responsive designs.