Most would admit to a degree of fascination for the enigmatic, the people who seem to keep at least a part of themselves hidden. Being mysterious ourselves can be fun too. We have great fun in hiding behind masks or in costumes. Say the word Halloween and watch the delight of not just children but grownups too as they plan the transformation that will disguise them to the world. Keeping one's identity a secret while trying to uncover that of others can be an entertaining diversion, our masks creating a level of anonymity.
The notion that there is more here than meets the eye works with image editing too. Just as the Halloween mask hides certain facial features, the Layer Masks in Photoshop will do the same to an illustration or photo. They allow you to reveal only what you want to see, without erasing or cropping, so that returning to the original image is as simple as removing a mask at midnight.
More on how they work can be found at these great sites:
Understanding Layer Masks in Photoshop
Photoshop's Masking Layers
Using Layer Masks With Photoshop
A Little Bit About Photoshop's Layer Masks
Remove Complex Backgrounds With Layer Masks
A Beginner's Guide to Masking in Photoshop