Was popular a number of years ago for building your own sinking fly lines. I guess they may still be popular for some applications. Coupled with a shooting head fabricated from a high density line such as LC13, it offered a method to make relatively long casts with a line that would get your presentation deep in a very efficient manner. Was popular on the Roanoke River during the striper run when the water was up and running strong and you wanted to target fish in 20 ft of fast moving water.
Though I've built them and used them, I'm with EvBlue when it comes to those types of water conditions I'm going with the spinning rod.
As far as what you can use it for, depending on how much you have available it could be used for flyline backing on a reel used for sunfish, crappies, etc where you don't expect to need alot of backing.