Question What does this cable do from the computer case do?

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It says USB but all the USB connectors I have on my MSI MPG B550 Gaming Motherboard are 9 pin connectors
this board appears to have a 4pin JUSB header on the bottom left corner.
you can easily just check the manual for all available headers.
my case has only one fan and no connector to connect to the motherboard
any fan has a cable attached to some sort of plug/connector.
just follow this cable from the fan and you will find it's plug either pre-attached to a case fan hub or hanging free somewhere.

this board has at least 5x 4pin fan headers available.