Why do many photographers add black background and white edge to photos in post-production?

I think post-production is to improve the shortcomings in shooting, such as exposure, depth of field, color saturation and so on. And some unnecessary factors that affect the overall composition and the theme of the photo. I usually use ps for photo post-processing. The best is the curve in ps. You can adjust the brightness and color according to your personal preference. For photography, the most difficult thing to control is color and exposure.