I always thought it was to make it a more consistent negative pressure. just like a man-made water reservoir... it collects the floods and makes the stream after the dam more steady.
it might also just be there to add more volume of negative pressure... I'm driving my mom's chevy beretta 3100 (beetle is parked 'till I replace the flywheel...). its multi-port fuel injected. I was experimenting with it tonight after dropping my girlfriend off at her dorm building. When I pumped the break pedal while the car was at a stand still, at idle, in neutral, the rpm actually jumped slightly. as we all know, gasoline cars get their vacuum from the throttle body, so when the vacuum goes down (approaches normal, ground level atmosphere), via the throttle valve, more fuel is introduced to keep the air-fuel mixture stoichiometric. So the brake booster might take quite a big bit of volume of air to operate.
but thats just my own understanding of it. my two cents
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