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Agreed, it is there as a reserve to the vacuum pump.

If the turbo, egr and brake servo all 'want some' at the same time then the pump could get overwhelmed.

The other main purpose is to provide a reserve vacuum for the shut down valve / air flap which lurks near the egr. Obviously as you shut down the engine, the vac pump is not running and could not provide vac to this, the reservoir does and can.
 
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