Basically, Bokeh does simple DOF calculations for you by showing you the amount of distance you have on each side of your focal point.

For instance, if I am shooting with my 5D and an 85mm lens at f:4 and I am 10 feet from my subject, my entire focal rang is just 8 inches from start to finish. Of course at the focal point it will be the sharpest, but when printed at a decent size of will still appear as sharp during that range.

There could be a number of uses for this. Just two I thought of off he top of it head were:

1. When panning or shooting a moving subject at long focal lengths, this tool would be great to help keep the probability to have a sharp photo high while managing the need for good exposure.

2. If you are shooting a particular size bug and want the entire bug in focus, you could use this tool to adjust your distance and aperature accordingly.