The sheepshank is probably one of the most under appreciated knots. It's fatal flaw is directly responsible for its lack of popularity: if the knot looses tension, then while thing comes undone. 

People usually tie a knot and expect it to hold until they release the knot, but the sheepshank will not do that. It will hold as long as the line has tension; but as soon as tension is lost, the knot falls apart. 

The sheepshank is used to take slack out of a line or to make a line shorter. As long as the line is pulled tight, it will continue to hold and do its job wonderfully. 

Knowing its flaw is important in deciding when and where to use it. If you shorten a dock line with it, you will be sorry! But is you shorten a line that is always under tension, you will be very pleased.  

I use this knot to take out the slack on my backstay adjuster. When I tighten the backstay, the adjuster line goes slack. A well tied sheepshank will take up the slack and will also release very quickly is I need to ease the backstay in a hurry! 

I must admit, this is the only place I have ever used a sheepshank on any boat, but I still don't think this knot should be cast aside as a worthless or useless knot.