Preconditioning, warming the cabin (and often the battery) while the car is still on charge, means the heavy initial heating load comes from the grid rather than from your battery. On a short winter trip, where heating is a big share of the energy, that noticeably protects range.
Our model halves the cabin-heating draw for the first part of a preheated journey, which is why the journey twin has a preheated option. On a long motorway run the benefit is proportionally smaller, because heating is a smaller share of the total.
Try the journey twin on any car page with a cold month, toggling preheat on and off to see the difference. The preheat assumption is documented on the methodology page.