Whether you remove nitrogen or add oxygen, regardless, you get the same air fill. Example, if you want a 32% fill, you set it up for 42% whether you use either method.
Perhaps you were asking what the advantages of Nitrox was over "regular air".
Basically, diving Nitrox lowered your nitrogen levels in your body which increased your "No Decompression Limits" which means you get longer dive times. You still
use your air/gas consumption at the same rate regardless of what you breath.
Some will say though that diving Nitrox will not make you as tired as you are
breathing more oxygen into your body. I am one of those who also agree
with this. I'm not as tired after I dive nitrox.
The real advantage of Nitrox is if you're doing deeper dives (over 60 feet) and
doing repetitive diving, like you would on a Liveaboard where you might easily
do 4 to 6 (or more) dives per day. Nitrox keeps your nitrogen levels down
which in turn increased your NDL for these repetitive dives.
One last benifit about the Nitrox class is that if you feel you didn't spend
enough time on your dive tables in your open water class, then it's a great
class for tables because you work them a lot in this class.
hope all that helps.
-Mike