It has no effect once the engine is running. It's the zero point Bosch used for the base position a in figures 12 and 14 when the engine is off. The air plate is at the narrowest point, and the fuel distributor piston is blocking fuel flow.
If the plate were set above that position, fuel could flow with the engine off due to a higher starting point of the plunger. If set too low then the plate would not be at the narrowest part of the venturi, it might leave a gap allowing for air to bypass the plate when starting, perhaps preventing the plate from lifting during startup.
The screw (4) on the small arm adjusts the fuel distributor piston to get the best fuel air mixture when the engine is running.
