They are usually made from fabric; while it's possible to make
a breathable mesh from most materials - such as pvc plastics - the
technology lends itself more naturally to flexible materials like
natural and manmade fabrics. As the name suggests, a breathable
mesh is designed to let the foot breath more easily, and cut down
on sweat and the build up of bacteria. A breathable mesh is usually
integrated alongside more stable materials such as leather and suede.
Without the leather/suede placed around the toes and heel, the mesh
fabric would be incapable of holding the foot in position, leading
to injuries such as a strained ankle.