Because at that point, your head becomes the "third leg" of the tripod of stability points holding the camera steady.

Until I read this thread, I didn't understand why it was so important to have a viewfinder for truly professional high quality photography. Now I see why it's so important. When you're talking about detail levels in a photograph that will stand up to serious scrutiny, and you're in a situation where a physical tripod isn't an option, then it's your only choice for a truly stable shooting platform.