The itinerary is used when working with moving venues, such as tours or round trips.
Compared to a venue a itinerary can have one or multiple stops where one can board / embark or unboard / disembark.
When the venue is not sufficient/flexible enough to define all of your slots and periods or routes with timetables you can use the Itinerary.
this will allow you to set multiple versions of a schedule to work together and create a complex time schedule with slots and capacity.