Having a person or resource marked as never booked allows bookings for the same time for different services.
E.g. Salsa (service) and Rumba (service) both happen at the Main Hall (resource). If Salsa is booked at 7pm, this means Rumba cannot be booked at the same time as they both occur at the same resource (the Main Hall).
Having 'Never mark as booked' turned on eliminates this problem so both services can be booked for the same time.