Attic Fan Louvres ?

Hello,
Regarding the exhaust attic fans one would put in the end of an attic in a Colonial style house:
I see many of them have louvres that open automatically when there is an air flow thru them.
Why would one want louvres at all ?
The only reason I can see is to keep (some of) the rain off the electric fan motor.
Is there any other reason ?
Sure seems like it would be better to have a constant air flow thru them in the winter, e.g., when the fan is not working.
What am I missing here ?
Thanks, Bob
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You are comparing gable mounted attic ventilators (no shutter), with whole house gable mounted fans, which are considerably larger and move much more air

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