I've known this since art school—almost all our works at exhibitions were framed with a mat. Back then, they were cardboard inserts. Now, in digital form, they are just borders, and in my opinion, they serve the main role: helping to focus on the image.

Mat (French passe-partout, literally — "passes everywhere") is a term that refers to a sheet of paper, cardboard, plastic, or other flexible material with a cut-out hole in the middle. It is used for framing a drawing, watercolor, engraving, or photograph.
A mat emphasizes the composition of the photo, adds depth and "air." With its help, you can draw attention to important elements of the image, color scheme, or shape.

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