Perhaps this might help
There are several components to Rasa core. It is perfectly okay to not use rasa core if your case is simple but as your usecase extends adding more and more conversations which overlaps with other conversations, it becomes almost impossible to manage, Rasa core determines based on examples what to do next , the next part could be a text or an API call you would like to make, you could even add a preprocessor(language detection) before calling the right agent, this comes in really handy.
I wrote a deep dive on how rasa core works sometime ago
Rasa Core Deep dive