What is this spring between the fork and the frame? Sometimes on a bike I see a short spring, most often covered in rubber, connecting the fork with the frame: 

What is the purpose of it? The only thing that comes to mind is stabilising the fork so it does not wobble to the sides so much. Is that its purpose? Why do some bikes have it?
The purpose of such a steering damper is to stop the front wheel from turning when using a (two-leg) kickstand or while pushing the bike. Not much use otherwise.