This is very useful feature in winter - start your car, turn recirculation on and go drink coffee at home. When you will be back, your car interior will be already warm and pleasant. Turn it off and drive. Recirculation speed-up interior heating at least two times, if not more.

Same should work for cooling too, in hot climate.

Meanwhile, if you are not aware, usually this recirculation intake is under dash at front passenger side. So, using it while driving, you will not only have low oxygen, but also sniff fromt passenger feet, shoes and all what had stuck to them. :)