Layers are useful because they let you add components to an image and work on them one at a time, without permanently changing your original image. For each layer, you can adjust color and brightness, apply special effects, reposition layer content, specify opacity and blending values, and so on. You can also rearrange the stacking order, link layers to work on them simultaneously, and create web animations with layers.
Layers are like stacked, transparent sheets of glass on which you can paint images. You can see through the transparent areas of a layer to the layers below. You can work on each layer independently, experimenting to create the effect you want. Each layer remains independent until you combine (merge) the layers. The bottommost layer in the Layers panel, the Background layer, is always locked (protected), meaning you cannot change its stacking order, blending mode, or opacity (unless you convert it into a regular layer).