Providing horizontal motion for absurdly complex dance moves.
Megan Garber delivered a pocket history of the invention of the gym machine today. But she left out one key use for the treadmill: providing horizontal motion for absurdly complex dance moves. So, I'm filling in that gap by posting OK Go's 2009 video, via Steven Lehrburger.
Also: this is a lesson in technical affordance. If one can do something with a machine, people will figure out how to do it.
Lastly: I want to go to the gym where this is one of the fitness classes.