The UV light sterilizes the water. Even though larger particulates are removed, the water can still be contaminated. From Wikipedia:
Ultraviolet germicidal irradiation (UVGI) is a disinfection method that uses short-wavelength ultraviolet (UV-C) light to kill or inactivate microorganisms
Never had it tested but we are still here and the water has never made us sick so I guess its working. The cartridges do most of the work. They remove bacteria and organisms from bird droppings for example. These nasties are actually quite large (relatively) as far I know, so a fine cartridge will actually take them out. I'm not actually sure what the UV does, considering that its 'after' the cartridges.
A guy on the tv show Survivor once said all he had to do was strain it through 5 t-shirts and it was good to drink.
Pretty sad state of affairs if you cannot drink the water straight off the roof , I could bottle and sell the water from our house tanks .
No birds in your area then?
Of course there are Mate. Possums too. (and why they need to rattle around on the roof at midnight is a mystery... there's nothing to eat up there!)
As I've said, I, and my tenants, have used and drunk straight roof-supplied rainwater since '98 with no problems. If I ever think there's a problem I'll simply run drinking water through a Brita or put the kettle on.