We had the Max air covers installed over the factory roof vents before we took delivery. They serve a few purposes.
The factory crank up vents cannot withstand the wind pressure of driving down the highway if you forget to close them as part of your departure checklist. They're flimsy plastic and usually only have the crank arm in the center of the vent, and will split the vent cover as it wobbles while travelling.
Also, with Max air covers, we can leave the vents open 24/7 all year long. No rain or snow or real dirt can get inside because of the mesh screens on the factory vent AND the Max air.
BTW........just like everything, the Max air II's provide more air circulation than the originals, but either model is better than none at all.