USB Host Shield DEV-09947, purpose of power Vin switch

Could someone explain the purpose of the power switch on the USB Host Shield? The shield does not seem to operate with it turned off, regardless of power source for Arduino, USB or external 12v.

Not sure if this is related, but as is noted in the comments on the productpage , the Arduino Uno does not seem to supply necessary power to some devices attached to Host Shield when Uno is USB powered.

Thanks for info.