What are swivels for?
When you get washed, it is not uncommon to be rolled underwater, and get tangled up.
By adding two swivels, you avoid getting tangled up. So when you surface you are ready to paddle from the get-go.
What’s a rail saver for?
Getting washed will often involve the ocean applying unusual forces to your equipment. In this situation, the rails around the leash plug of your board are especially vulnerable to a ding or crack. By adding a (wider) railsaver, the pressure on your board is spread more evenly, saving your rails in the process.