Since you can tie as many butterfly knots as you like on one line, the loop is an easy way to provide several anchors for any given reason. This can be particularly helpful for search and rescue ...
and is relatively easy to untie after it has been loaded by a fall. To tie into a harness with a Figure 8 Follow-through, tie a Figure 8 knot in the rope approximately 3 - 3.5 feet (1 - 1.5 meters) ...