Heater valve.....

Can anybody tell me what the heater valve does on my 190e 1.8 ?........I've just replaced iy 'coz it was leaking and can't figure out why it is there as
the heat controls on the dash turns the heat up and down without it........where does the vacuum come from to operate this valve ?...when does it shut off the flow of water out of the heater?
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It's ok I've figured it out........
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