Sub Events can be used for setting different times or locations for the Main Event. For example your main event may be visiting a museum and the sub events can be time slots for visiting that museum such as being able to book a 9-10AM session every Monday or a similar arrangement.
When creating a Sub Event all details will be copied from the Main Event except for the Event ID, Fundraising Target and the Feature check boxes. All fields can be changed in a Sub Event and there is no requirement for those fields to match the main event.