Either we see a picture in isolation or as part of a collection in a room/book with others. Here we generally see pictures one at a time. When seen alone against one edge of a highly formatted page, as in OPF, it's composition, how things are weighted, what is free to move and expand, are all limited.
When a picture has white space around in all directions, things that are incomplete can continue beyond the borders. When there's movement, we can allow more, if it has the white space to also occupy. Next, as a bonus, all other things around the picture are not part of out attention. Even a simple blue line on the border of a post can take the eye away from the subject been offered. With white space, we remove such distractions. There's now no interference in experiencing the photograph.
A thick black border with detail structure in it imprisons a picture. nothing can go on outside of it. A black free space, OTOH can be an added resource in the composition since it can hold the unknown.
So, to get the most power, continued experiences out of a picture, add white space around it. The exception is in those instances, where, for example, one wants to show something where its very position at the edge would help in the esthetics. An example might be a depressed sad withdrawn individual, sitting on a doorstep, head in the arms, forlorn. Place at a lower stuck against the left edge of a page would help to deliver this rejected hopeless feeling with more limited choices
