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The petcock is designed to open and allow a given amount of fuel to the carburetor to provide the optimum air/fuel mixture based on the amount of vacuum the carburetor generates which is controlled by the throttle valve. If you leave the petcock on prime, the fuel to air ratio would be too high(rich). The diaphragm regulates that flow. Also, the petcock set on the ON position allows you to only run about 2.8 to 3 gallons of fuel out of the tank allowing you to have reserve fuel available when you make the switch to reserve so you are not left high and dry on the road. It does act as a shut-off as well on either the reserve or ON position as there is no vacuum pulling fuel out when the engine isn't running.
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