Carby heat
Several possible reasons, some, all or none may apply to a specific AC. The hot air is sourced from a baffle around part of the exhaust. This MAY be vented to free air with heat off so as to cool the manifold. Selecting part heat could block the venting, but not feed the hot air to the carby resulting in over temped ducting and hoses.
Also it could restrict the flow into the carb, thus cutting max power, or the swirl from the flap can result in very strange performance and mixture problems.
This is what test pilots are payed for!