As I said, it reduces problems of SSTO (Single Stage To Orbit). The problem of SSTO is low payload mass to the size of the rocket and fuel necessary to transport it. The main solution to this is having multiple stages. Which comes with its own problems -- multiple engines, complexity, more points of failure, etc.

But in the meantime SpaceX learned to reuse the stages so suddenly the problems are no longer as much of a problem as they were before.