Have you ever needed to make something look like a local drive that wasn't? or wanted to use a filesystem based tool to save to a database or post something to the internet?
If you are a linux user this is not a really difficult thing, but in windows land there has not been a really approachable way to do this without getting down into the weeds with some C driver development tools.
This is where Dokan comes in.
Dokan gives you a driver that handles all of the ugly low level bits and lets you develop the functionality you need in a higher level language. I have been playing with it for the last few days and by what I have seen so far, I think I would have killed someone to have this a couple of jobs ago when I was dealing with insane amounts of jpeg images all the time.