Excerpt from blog post:
"Of all the texturing methods I learned during the early years this was one of the most useful (second only to mix maps). Using an alpha map allows you to easily control which parts of your model appear in a render. It also affords you greater control over materials used in your scene through layering, adding extra detail to low poly models, and allowing for translucency in textures.
Plus you get to feel like a ninja when using such a technique in front of your less experienced colleagues (and less like a noob when it's used in your vicinity)."
To keep reading go to http://www.shadesofadream.com/blog/tutorial/alpha-maps-how-to/