How an empty leg arises
An operator needs to bring an aircraft back to base or to a new mission. The empty sector can be offered to a client whose needs match that route in that time window.
When it really makes sense
When dates are flexible, origin and destination match or are very close to the repositioning route, and the aircraft is consistent with the number of passengers.
Limits to be aware of
Availability is not guaranteed until confirmation, and timing changes or cancellations are possible if the operator has to reassign the aircraft to a full charter.