Why empty legs happen
Private aircraft do not always have passengers on every sector they need to fly. Repositioning to the next trip or returning to base creates inventory that may still have commercial value.
Operators may choose to sell that sector at a reduced rate rather than fly it empty.
Why they can be cheaper
The saving exists because the aircraft was already going to fly that sector. The operator is monetising a flight that would otherwise move without passengers.
That does not mean every empty leg is a bargain. It means the pricing logic is different from a bespoke charter request.
Their limitations and cancellation risk
If the underlying booked trip changes, the empty leg can change or disappear as well. That is why empty legs should not be treated like guaranteed scheduled services.
They can be worthwhile, but only if the trip is flexible enough to absorb that reality.