Layering in art is used to make a design more visually interesting and can be used to make effects that wouldn’t have been made otherwise. Layers have been created through aby combination of words, lines, shapes or planes.
The book Graphic Design: The New Basics by Ellen Lupton and Jennifer Cole Phillips states how layering was used physically to develop photos but have since moved to things like Photoshop and other digital software it also describes how layering can give certain elements in a design more importance over others based on the way they are arranged. In the example from What It Is by Lynda Barry shown above you can see how layering can be used to create a cohesive and interesting design. The page looks like a collage because of the different layers being cut and pasted into the page along with these of color within similar layers like the images near the middle of the frame like the lock, matches, fan, etc.
Another example of the use of layering in a design is from The Best American Comics 2016 also shown above this picture uses layering to show how different the room in the photo was throughout the years. It uses layering to show the years in the upper right corner along with different frames of different elements that look to belong in the room arranged throughout the design.