Having a long length of line ready to use at short notice is always a good idea when cruising. You never know when you may have to run out a long mooring warp or set a kedge anchor. The trouble is that such a seldom-used line often ends up under piles of gear in the cockpit locker. This is a bad arrangement, because when you want a long line you often need it right now. You don’t want to waste precious time burrowing for it.
This sailor has probably learned that lesson the hard way, which is why he has installed this spool of strong webbing on the stern rail and has made sure that it is ready for instant deployment.