Integrator can be used as an Enterprise Service Bus, serving requests from third-party applications. Basically, you can build an API which can be used by other applications to send a payload to Integrator or request enriched data from the Integrator, without writing a single line of code.
The event-driven flows are the extract data from source, transform, and load in destination flows where either source or destination is a listener.
The other name for event-driven flows is user-defined APIs. Read how to create user-defined APIs in Integrator.