The very first tool that photographers have to drive the attention where they want this to be in an image.
I think it even is a fisiological feature of the sight: it is impossible to look at something blurred without feeling strange and, sometimes, even feel some kind of pain.
This way, the photographer forces the audience to look where he wants them to.
This very simple video uses the Dof to drive the attention and through this read a little poetry.
