There are all sorts of interesting things inside your iPhone 4, but you can't see them because of that pesky back panel. Sure, you could use the phone without a back panel, but that wouldn't do your phone's longevity any favors. This decal is a smart trick for getting the same effect without killing it.
For a mere $10, you'll get a decal that reveals exactly what's inside your phone without actually showing what's inside your phone. A neat trick! And probably one of the most apt back decals around.