It's designed to keep your chain under tensionto allow rear derailleur movement with the rear wheel removed. This is useful for a number of reasons.
When cleaning a bike on a stand it's much easier to remove the rear wheel first. You can get to the brake arch to scrub out and inspect the pads - impossible with the wheel in place - and properly scrubbing the drivetrain down is harder and takes longer with the wheel on the bike. Also, access to the inside face of the rear derailleur is much easier with the wheel out of the way.
But removing the wheel drops the chain, making turning the cranks likely to damage the chainstay, and you'll also end up jamming the chain into the chainrings. Enter the dummy hub, also known as a sleeping hub.
Simply remove the wheel, reinstall your axle, slide the chain and chain keeper over it before slightly nipping it up and done.The unique design allows the bike to be shifted.