I'm not sure about photography, but this topic of using window mats can go all the way back to the 13-1400's when artwork used to be displayed in a salon, were all pictures were hung side by side, from ceiling to floor with no wall showing. At this time, nearly all paintings had what I call a "built in perspective system" which took the viewers eye directly into the image and told you exactly what to look at.
At time went on, artist started changing the compositions of their images, making more use of the area that would exist outside their painting, where the viewers mind could emagine a whole scene, and the painting just being a tiny detail of that scene. By doing this, paintings never looked good when hung next to each other so artist started using super wide and thick frames to isolate the image. Soon, a larger space was needed around the image, so mats were used.
By the time photography was invented, I believe artist already knew that images needed space to breathe, and matting/framing was part of the final displace of the piece.
Of course, over the years, there has been people not using the frame and mat, and creating images that have no perspective, but rather an all over image. For example, the spatter paintings of Jackson Pollock. However, in this case, it was about how the final piece really communicates with the wall and becomes part of the gallery space. A frame was not needed nor wanted.
Hopefully that clears some stuff up for you!
Regards,
Ryan McIntosh
www.RyanMcIntosh.net