Map projections are necessary for creating maps. A map projection is a method of representing the surface of a sphere on a flat surface (plane, paper).
Basically, the world is a sphere (the shape of a ball), and a map is a flat surface. It's difficult to take a sphere and attempt to form it into a flat surface, since a sphere is a curved object, and a map is completely flat.
All map projections distort the surface in some fashion. Depending on the purpose of the map, some distortions are acceptable and others are not; therefore different map projections exist in order to preserve some properties of the sphere-like body at the expense of other properties. There is no limit to the number of possible map projections.
World map projection, Europe and Africa centered.
World map projection, Americas centered.
World map projection, Pacific Rim centered.
World map projection, Khartoum (Africa) centered.