AFAIK in my Dakar unless I deliberately press the recirculation button it will automatically switch to external air once the fan speed settles as it approaches the set temp.
As Chris mentioned it is for economy and to reach the set temp faster. However if you dont touch anything it will switch to outside air itself.
I always run on recalculated air to save fuel and to reduce bad odours from outside.
I Always switch AC off and to outside air as I wait for turbo to cool down. Apparently better for compressor to start vehicle with AC off. Also blows things clear before shutting off then.
Turn ignition to ON position
select FRESH button (in lieu of recirculate)
Press and hold OFF and AUTO for 10 seconds
Turn ignition OFF while holding down the 2 buttons above.
Now it should start up on FRESH new time