The ability to focus is the ability to influence the future

Focus in photography enables the photographer to direct the viewers attention to a predefined subject in order to influence the viewers experience of observing the photo.
Therefore the photographer takes control of the perception of the viewer and uses it to influence the subjective interpetation of the photo. 
Focussing requires a technical skill that needs to be used decisively and quick in order to capture the moment. Before focussing, the photographer needs to analyse a predetermined non-existing image that the photographer is able recognises in the scene before him.