Swivels are great for tying between the tippet portion of the rig and the standing end of the leader. When you tie two flies together and add split shot the rig often twists and spins in the air resulting in a knotted mess. The Swivel eliminates a lot of would be tangles as it allows things to twist and spin in the air without becoming tangled up into a mess.
A must for high water nymphing where you are using heavy rigs, multi fly rigs, or rigs with a lot of weight/density.
An added bonus is the eye of the swivel makes a fine eye for tying off a third fly /dropper.