Regardless of what image you are creating, it is likely that you will be able to group objects together that are similar distances away from the viewer. A simple example of this is to separate a painting into two layers—one layer for the background and a second layer for a person. In a more complex composition you may have a sky on a background layer, buildings on a second layer, and a person in the foreground in a third layer.
In both of these situations, the purpose of distributing parts of the image onto layers is to allow for changes that wonʼt affect the entire image. So in this example, if you decide to erase part of the butterfly, the sky remains untouched as it is painted on a differentlayer beneath.