I own one and am buying a second.
1. Allows the use of non-Poseidon hoses (easy field replacement when Poseidon hoses are not available).
2. Increases comfort and reduces jaw fatigue when used on a long hose (it allows easy adjustment of the 2nd stage position).
3. Allows safe and streamlined underarm routing for use on a necklace-mounted primary 2nd stage or recreational short hose.
The swivel is neither too lose nor too tight. It holds the adjustment well. I found the whole unit including the swivel to be very well made and well worth the cost. This is a highly recommended convenience for Poseidon regulator users.