Why is my furnace blowing hot air outside?
All gas furnaces require a flue to exhaust carbon monoxide outdoors, the reason modern units require a fan to do it is due to  the circuitous design of modern heat exchangers that extract more heat from the unit before exhausting it outdoors, natural  venting will no longer happen if the induced draft motor is not running, So much heat is removed these days they can be vented using PVC flue rather than metal and  moisture condenses making an acidic condensate that is drained just like the AC condensate is.
It is normal,if it were not so you would/ could die by operating the unit in that condition.