the riemann maping theorem tells you there are exactly 3 simply connected riemann surfaces, the plane, the disc, and the sphere.
it is always useful to know all possible objects of any kind.
since every riemann surface is the image of a covering ampping from a simply conected one, this says there are three kinds of riemann surfaces altogetehr, those whose covering space is the sphere, the disc and the plane. among compact riemann surfaces, it turns out the sphere covers only the sphere, and the plane covers only the surfaces of genus one, and the disc covers all the rest.
this gives the basic division of the world of surfaces into three types, parabolic, hyperbolic and elliptic. i.e. curvature zero, negative, or positive.