What are the preflow and postflow timer settings used for on the Miller 350P?

Preflow (PRE) is the amount of time that shielding gas will flow after the trigger is depressed and before the welding arc becomes active, with a range of 0-5 seconds. Postflow (POST) is the amount of time that shielding gas will flow after the arc has shut off, with a range of 0-10 seconds. These settings are adjustable in the TIMERS menu. (Page 27)