The following round fan is installed in a residential roof in a house in Pennsylvania. The roof also has soffit and ridge vents. The round part of the fan spins, but it is unpowered (by electricity). So what is this vent or fan called, and what function does it serve that is not served by ridge vents?
They're typically called air turbines or roof vent turbines. They can turn if the outside wind is blowing or if hot air is simply rising through them. They're a type of unpowered exhaust, just with moving parts. Typically you'll have ridge vents OR powered attic fans OR these. The advantage ridge vents have is they have no moving parts.