There's several ways to get a fatigued, injured or unconscious paddler back to shore. One of the ways is by towing. Giving the rescuee your leash is one way to do it if you don't have a tow rope. The downside of course is that now you're now leashless. Carrying a tow rope means that you can stay leashed up while still being able to tow the rescuee to shore.
Of course most regular paddlers won't have a tow rope on them, but it is a good idea if travelling far off shore, on expeditions, or for teaching classes. I've towed two students last year which included a woman who was too fatigued to paddle back to shore in moderate wind.
There's several compact two lines in the market which can be worn around your waist with a quick release buckle, or attached to your board via outfitting. Either way, you should have a method of releasing your end in case there's a snag, etc. Many kayakers I know make their own tow lines. I use Salamander tow ropes as they've held up in heavy conditions for years. They also make great lines for cooling your beer while camping.
- The best way to attach a tow line to another board is to it's bow/nose if it happens to have a leash plug or stick on loop (EZ Plug or NSI plug). But 99% of the boards out there have no attachments in this location. I'd recommend adding a nose loop if teaching.
- You can also have the rescuee hold the tow line end (usually a carabiner) or wrap it around the shaft of T-Grip of their paddle. It's best to have them sit or lay prone (flat) to keep them stable while towing.
- Loop the end around the widest part of their board and carabiner it around itself completing the loop, then the remainer lines up towards the nose and attached to you (make sense?) See Photo.
- For whitewater, you may consider getting a throw line/bag which is thrown from shore to a swimmer or pinned paddler.
* The rescuee's line should detach easily if there's a problem. Never tie either end in knot or permanently (unless you don't have a choice of course).
The photo is a skills class I taught with the Kayak Academy staff in Issaquah, WA in January.