Beside the rather generic introduction to pixel layers you can use them to create almost anything you want:

• To create panoramas, by 'stiching' several layers in order to create a single, larger image
• Stacking to
  • Extend the sharpness
  • Remove noise using several techniques
  • Extend the dynamic range of an image
  • Create composites
  • Create special effects
• To create an animation

This list is just to give you an idea of what you can do with the 'simple' pixel layers.