What is the purpose of UV in a UF purifier?

Purpose of UV + UF in a purifier


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? 


V Shrirama