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

Years ago it was used to alert the postal employee when they delivered mail that you had postage paid mail for them to pick up, and that’s the only thing I’ve ever known it to be used for. This was common practice in the past, personally I painted mine the same color as the mailbox and let it stick to the mailbox.

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

If you are mailing something you put it in the box and raise the flag. That way the mailman knows he has mail to pick up.

Incoming Outgoing

up when you have out going mail

To indicate someone from Russia has sent you a letter bomb