Studio Arcanis

I am planning to make a new black mirror and have noticed some people specifically prescribe a round mirror (and not any other shape). Does anyone know if there is a reason for this, or is it just for aesthetics? I have never had problems with a rectangular mirror, and i am sure most people in the past were thrilled to get any shape of useful object, but I was curious!

(Sorry for simple question and hope I didn’t miss the answer somewhere else here!)