The absolute easiest way to finish a fly is by stacking a couple of half-hitches at the head of the fly putting a dab of head cement and calling it a day. 
However, on smaller patterns it's difficult to get the precision needed by just using your finger. That's where this tool comes into play. You wrap your thread around the end and place the nozzle of the tool over the eye of the hook. Transfer the knot on the tool to right behind the fly and PRESTO you have a finished fly! 
There's also a unique post at the back end of this tool which prohibits it from rolling off your desk while tying!