Pocket ruler is a scrollable ruler that allows you to measure items that are longer than your iPhone/iPod.
Other scrollable rulers allow you to scroll through the ruler but are useless for measuring anything longer than your device because the start of the ruler disappears off the screen as soon as you start to scroll meaning you have no point from which to start your measurement. Pocket ruler overcomes this by using a simple but effective system of stationary and scrolling points which allows you to keep an onscreen reference to the starting point.
If you are still not sure why not try Free Pocket Ruler it is basically the same app with ads