Roof Purlins

I was wondering wether someone could tell me the need for roof purlins I am building a roof 3 degree pitch over a deck that is 3.5m out by 6m long, I am using 190 x45 for the bearers and rafters, what is the main difference with using roof purlins (70 x45) and the other method of just noggin it out with no purlins, other than purlins being the faster method, are their any advantages of having purlins. I thought maybe the purlin provided the roof with an air gap to prevent condensation.

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