What more can you say about the name? They like to say "Don't be Kinky; Switch your Swivel." Like the function of any hydraulic component such as the pump, the cylinders, the hoses and the fittings, if your hoses are going to turn one way or another, a swivel is a necessity. Not just swivels for hydraulic fluid.
Ever go to a self-serve car wash? A swivel turns the hose! Pressure washer? Keep the hose turning as you do your cleaning. Pulling a hose off any kind of hose reel? The water comes out of a spigot, goes through a swivel and into the reel where it connects with the garden hose. Then as you pull the hose out of the reel, it turns while the swivel keeps turning with it.