Useful for bike cleaning, and also for transport. I like the ones that go across the rear dropouts, like a false axle if you get the idea, and have a free running plastic pulley.
After a few years of riding many of us will have an old cream crackered rear wheel. You can cut the hub out of the rim, and I then use just one cassette sprocket in the centre of the body, using old cassette spacers to take up the extra space. Also ideal for transport as it prevents the rear ends being accidentally squeezed.