The double fisherman's knot (sometimes called a barrel knot) is used to join two lengths of line. It is one of the knots most often used in climbing as well as search and rescue.
Other uses include:
To back up a critical knot (harness tie-in knot or single-line rappel rigs). In these instances the running end is tied around the standing end of the rope so that it cannot slip back through the knot.
The ends of a necklace cord can be tied using the double fisherman's knot and,
This is the knot most employed when creating a loop for a Prusik Hitch.