The honeycomb structure traps dead air.
Dead air is what makes wood a natural insulator and prevents it from easily conducting heat or cold.
This means wood will not significantly contract or expand with the environment's temperature fluctuations. Door surfaces contracting and expanding at different rates is what causes thermal bow. The offsetting grain pattern of the individual pieces of wood produces a stronger, more stable overall unit. This is because the grains are multidirectional and will not all bend in the same direction. The veneer gives it the appearance of a single piece of wood, which is aesthetically more appealing.