What purpose does the Notification Flash/Email setting serve? The Notification Flash/Email setting (Settings > Sound &amp; 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.
The Notification Flash settings are supposed to enable/disable flashing the LED on the front of the handset for events such as receiving an email or SMS.  If your handset has no LED (my HTC Desire does, my ZTE Blade does not) it has no efect but the option is usually still present as it's a stock option.