In the case of comic book artists, a panel might use four basic layers. Well, with layers, you are still building up your image however, when you need to erase something or want to change something that's on one layer, the rest of your image isn’t in jeopardy. Traditionally, you have a piece of paper and you draw, building up your image. You might be asking why anyone would draw like this. When each of these are combined, you create the overall image.
If you were drawing a scene of someone sitting at a beach, you could break down the scene into layers or even layer groups.
Think of layers as transparent sheets of plastic that get combined, can be ordered and reordered, to create a picture. Do you want to import an image? The image will be loaded onto its own layer.
They are used a lot for complex drawing and projects. They help you organize the content that make up your image.