The double overhand is used occasionally as a stopper knot, tied in the end of the rope to prevent you from abseiling off the end.
It is more often used to tie loose ends off around the main strand of rope, or back up knots such as the rethreaded figure-of-eight or figure-of-eight on a bight.
It is also used as the basis for the double fishermans bend to tie two ropes together.
|Pass the end of the rope over itself...|
|... around the main part of the rope once more...|
|... and then under itself and through the first loop formed.|
|Pull the knot tight and set.|