Not sure as to the exact reason but most all diesel engines have seemed to have them. They most likely cause the air to rotate dropping out the large particles like a vacuum cleaner
You are correct , the fins cause a vortex action created by the incoming air which throws heavier particles outwards which are collected and dispensed via the rubber reed valve at the bottom of the air cleaner housing . It also helps elliminate punctures of the paper element by sharp objects such as grass seeds etc by the same action .
I always thought that when the air was being sucked in they would turn and act like a "turbo".. But I guess that isn"t right either!!!! Because when I installed it yesterday I tightened the screw up against the filter,, therefore no spinning!!!!!! All I know is that it went into the cannister,, and fit......