The Notification Flash/Email setting (Settings > Sound & display > Notification flash > Email) appears to have no influence on the stock mail app or gmail app. So does it serve any purpose?
In the stock mail app, even if Email notifications (Settings > Notification settings > Email notifications) are on, the flash doesn’t occur, regardless of the system wide Notification Flash/Email setting.
In the gmail app, the Email notifications (Settings > Email notifications) setting controls whether the flash occurs or not, regardless of the system wide Notification Flash/Email setting.