What is the purpose of UV in a UF purifier?

Hello,

I am told UF with a pore size of 0.01 micron filters out all living orgranisms. if that is the case, why do most companies use both UV + UF? What is the purpose of purifiers having both UV and UF in a single purifier? Is this just a marketing gimmick or is there something we are missing here? 

Thanks

V Shrirama

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