Robert, to add a point to Joyce’s note (specifically to answer your question: “Why would we use the free one when we’ve already bought a license to use the premium version?”). The free version is the base code for the PRO version; in reality, PRO is also just an add-on that extends the functionality of the free The Events Calendar.
So you can always run The Events Calendar with any of the add-ons no problem, but if you wanted some of those PRO features to integrate (ie recurring events, saved venues/organizers, etc) you’d want to activate PRO as well.
Hope that helps and please let us know what else we can do.