In the 17th century, sailors spent much of their free time crafting and repairing gear for the ship. Every element of a sailing ship required constant maintenance -- which is still true today.
Little on a ship wore out more quickly than its rope; our word "junk" was originally a nautical term for rope that was too worn to be usable. Sailors thus had ample opportunity to practice their craftwork and plenty of scrap rope to work with. The art of rope use, or marlinspike, grew out of this utilitarian reality.