The swivler attachment is very useful for the traffic handle configuration, allowing easier attachment/detachment to/from the collar or harness, while providing 360 degree rotation and swivel motion to avoid twisting during walks.  It's ESSENTIAL for the safety of your dog on TIE-DOWNS if the leash is attached to a COLLAR WHILE IN TRAFFIC CONFIGURATION.  

The Swivler attachment can also serve to convert your leash to  a double-ended lead.  This is extremely effective if your dog pulls.  The two points of contact provide an extra instrucitonal signal, while distributing the force for a gentler lead.  A must have training accessory!