A quick experiment to check out the kWh use for a standard pre-condition (30 mins?) at 1C ambient gives 2.9kWh!
Presumably, not preheating only saves that 2.9kWh if you then don't heat the cabin to the same extent when you set off driving.
This is where some cars are a bit sneaky with consumption displays, I think, because if you preheat, the consumption displayed will be better than if you make exactly the same journey and don't preheat but do then still heat the cabin when you set off. In reality, the same (or similar) amount of energy has been consumed by the car for the purposes of making both journeys.