I think that's the more 'technical' answer Jim...
In my 'stupid person' explanation....
It's added to collect the oil vapour that comes out of the top of the engine and gets sucked along the air breather tube... this happens to connect to the VIS valves on the ZS and cause them to 'foul up'. The tank (in theory) stops the oil vapour getting as far as the VIS valves and therefore reducing the chance of their failure.
I hope that maybe helps, but knowing me it may just have muddled you up more...
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