Besides helping you locate a restaurant for dinner or find out when the next bus arrives, the iPhone can also capture interesting pictures (with the, of course). But the application we're talking about here, the Viewfinder, is not for taking pictures. Rather, it's to help professionals frame their shots before pressing the shutter or rolling the film.
Viewfinder (available on the iTunes App Store for $7.99) makes use of the device's onboard camera and lays grids on it to simulate the focal length you specify. If you're using a 50mm lens, the screen shows the scene and a box with the view you would get on your camera. So, instead of squinting through the snapper's viewfinder, you can now do it on your phone first and then reposition your shooter for the shot.
If you're using a medium-format digital camera, you'll have to pay $7 more for the Pro version which supports more models.