What is the importance of reproductive isolation in speciation?

Speciation is when a new species starts to exist, right? Since reproductive isolation is when members of the same species stop breeding with each other, it is exactly what would stop that potential new species from prevailing.

This can be caused by mating differences, sterility or environmental barriers that eventually lead to the adaptive splitting into two species. However, reproductive isolation is not sufficient but internal barriers to gene flow are required for speciation to evolve.