Photography is a medium with a frame implicitly built-in. The edges of the image isolate a section of reality, and the frame exerts a constant, if unseen, force on our interpretations. Not all images are the same shape, and different formats bring different shapes and image edge ratios.
Hold a small print in your hand and it can quickly disappear into its surroundings. This holds especially for the print with little or no border: with no protective visual buffer, the surroundings come rushing in and the print lost to anonymity.
Give the print some room, some visual, protective space and everything changes. Suddenly the image is contained and exerts its own reality. Our eyes are held much more effectively within the images’ borders and the photographic illusion - that we are looking through a window on the world, or more strongly, that we are occupying that world - holds sway.
This is a matter of aesthetics too. The visual attractiveness of a piece can be given a huge boost when it is placed in a mount (and arguably more again in a frame). I order my mounts online and have them cut to exact dimensions to suit the image. I carefully match the mount colour - and remember different whites have very different ‘colours’ - to the paper colour. A good mount is somehow not noticeable in itself, but surely is in its aesthetic impact.
The only way to appreciate this is to try it. Take an unmounted finished print and treat it to a well-cut mount. Observe what happens: put it on a table, step away from it. Take things further and find a discrete, proportionally matching frame. Try it on the wall. Step away. Return to it. It is my wager you’ll love the image more.