For best practices, you should choose "Add Occurrence" if you set your event up with recurrence rules and you want to add an extra day or two. You can also add an occurrence when you want to add a "one off" occurrence. For example, if you've set your event up to recur every Thursday and you would like to add a Friday, you can do so by going to the Availability tab and choose "Add Occurrence".
When adding a large event that has many components to it, you can add multiple schedules within that event. A good use case would be for a wedding. Let's say you are scheduling an event called "Barton & Andrews Wedding" and you've added all of your items, but you want to add a "New Schedule" for the rehearsal and one for the reception, with different date/times, items, etc. You can add a "New Schedule" for the rehearsal and another for the reception.
All of your events with occurrences and/or schedules are visible on your dashboard, as well as, on the Availability tab of the event.
I hope this information was helpful! If you have any questions, please reach out to firstname.lastname@example.org. Thanks!