The white border give that feel of a classic hand printed photograph. The printing easel would mask the paper around the edge of the sheet of paper and leave the border. Very early in photography you would also see black borders on prints when they were contact prints from a negative held flat on the paper with glass, and the clear border of the exposed part of the negative would produce a black border.

In the printing industry and publishing/press fields the images would often have a rule line around them, a thin black line to especially stop a bright image bleed into the white paper and help the image stand out. A white border will help an image stand out from a dark background.

Any border helps to isolate an image from it’s environment like a frame on a wall. It give the image it’s own space and helps people to focus and concentrate on the subject. It contains someones attention. In a context like Instagram it hopes to stop you scrolling away and spend a few seconds appreciating the work. It certainly got your attention.