Why do some cameras have to square matte box?

Nearly every camera lens comes with a round or square, or petal-shaped matte box. And different matte box length of the lens is not one. Sometimes, you find it troublesome, simply throw them at home. First, they are useless in the end? In everyday life, when we encounter very intense sunlight, we cover the eyes with our hands above our eyes, blocking the reverse sunlight and letting the light of the front scenery into our eyes, so that we can see clearly. Wearing a matte box lens, like with our hands covered in the eyes above. The picture below is matte box backlit photo effect: Matte Box SK-MB1 The Sevenoak SK-MB1 DSLR Matte Box with Top Flags for 15mm Rods from Sevenoak is suitable for use with a wide variety of DSLR cameras and baseplates with 15mm rod system(cameras, baseplates, and rods are not included). It is constructed from high-grade ABS plastic, and the included flag is made from aluminum. It is used on the end of a lens to block the sun or ...