The asshole answer: So people can think for themselves and not have to worry about trying to make every subject fit in to some 'ideal' proportion.
A more useful answer (a mere rewording of what everyone else has posted
): The concept of the golden section thing has been used for artists and architects for a long time, and can be found in nature. For some reason, people find it a pleasing format, and it can be if used right. But if you go crazy trying to make everything fit in to that one shape, then you're actually hurting your photography. Use it for the subjects that need it and ignore it for the subjects that would look best it a different format.
I think having it etched on to your focusing screen will make you try to use it every time the camera is pressed against your face