Quote - There are so many advantages of working with layers that it's difficult to summarize them, but here are a few things you'll love:

• You can separate parts of the image and edit them without affecting other parts of the image.
• You can use layers as guides or reference without including them in the final image (just make them transparent before saving the image).
• You can safely import new images to add to the composition. Move the new image around, resize it and do whatever you like without damaging the original image.
• You can create multiple versions of a layer and experiment with different effects. You might like to keep an original image and make a separate layer to work with, so you always have the original image layer to fall back on.
• You can apply filters and effects to layers independently, e.g. drop-shadow, colour adjustments, etc

Just to add if you also want a layer to be under another layer for example shadowing, just move the layer down under the layer you want the shadowing to appear under.