Edge finding is one of the most important steps in any complex milling project. Edge finding allows you to very accurately locate the position of your workpiece after it's been mounted on the stage. This is absolutely essential if you need to accurately mill features between re-homing, for locating backside features, or when modifying an existing part.
In large milling machines edge finding is done using a special tool called, you guessed it, an Edge Finder! Unfortunately it's pretty rare to find an edge finder that will fit in the Othermill. See the wikipedia article on Edge finders: http://en.wikipedia.org/wiki/Wiggler_%28tool%29
While Otherplan has locating feature for getting pretty close it's very hard to much more accurate than a few thousandths of an inch. I still use the built-in edge finding for locating a new rough material within the mill.