Why is Kaneki's fighting ability improved when he put on his mask? In volume 8 chapter 72, Nishio mentions to Kaneki that his abilities will improve if Kaneki puts on his mask.
As far as I know it's just a mask (with no special power or anything). So why does it increase his fighting ability?
The mask covered Kaneki's nose and mouth. This helps him suppress his urge to eat the opponent by reducing his ability to smell. Remember that Kaneki's ghoul ability is originally Hanazawa Kana's...no wait...Rize's, nicknamed the Glutton for her great hunger.

By suppressing this urge to eat, he can concentrate more on the fight as he's not worried about getting carried away and going berserk.