Different map projections are necessary because they were created to address different situations. Cartographers are the professionals that elaborate map projections. They do it in order to have a visualization of the Earth Surface.
We can’t have just one map projection because there is non-single map that creates a perfect image of the Surface of the Earth. That is why cartographers need various maps to overcome the problem of distortion. It could be areas distortion or distances distortions. Different maps help to meet distinct needs.