Two very good uses for a long very long 15' or 20' cotton training lead.
Teaching come outside. or the Recall command.
The other is Sit stay or Down Stay.
Pretty cheap to buy too
The length is used fully after a reliable come or stay is taught at say 6' then 10' then 12' then right out to 20 feet.
You can then move outside a fenced in area, use a handy tree to wrap tie off the long lead, and practice sit stay, come etc with
It helps keep your dog safe, it they bolt away on you! Can only go 20'