What is the 'flag' on a USA mailbox for?

Never seen any mailboxes here with that feature.

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it's to tell the homeowner they have mail. so when the posty puts mail in it they flip the flag up...when there's no mail they leave it down...that way one doesn't have to waste their time checking the mail....i'm just surmising that as i'm canadian and i've never seen it here before but then again i'm city folk