The reason is so as to cover as much of your eyes as possible. However, both the surfaces of the lens have the same curvature so the cancel each other out behaving like a normal piece of glass instead of a lens.
Sunglasses are more of a fashion statement than a shield against sunshine. Nowadays, people pay much attention to their fashion function. The shape of sunglasses has something to do with their appearance. Most of us want to make them looks chic, so they always would like to wear sunglasses to style themselves. Any place that you find a person wearing sunglasses, it is just as possible to find a person not wearing one, and just as comfortable under the conditions. So, while in some cases they might round up on your face and end up offering you more of a shield against sunglight or dust, there are no strict lines drawn. The curvature of sunglasses, just as their straight and squarish look, always has an aesthetic appeal attached to it.
A curve is part of a circle, it is also part of a sphere, and it is also part of a cylinder. In optics, a curve is called a curvature. It is the curvature in your eyes that makes them focus and produce a clear picture. It is the curvature in the lenses that gives them the optical power to focus or to change the focus in your eyes. it is this curvature and nothing else. There are no molecular changes in the glasses, there are no other physical or chemical changes. The differences in lenses are differences in curvature. But not all sunglasses are curved though.