Do you ever wonder where your money goes? Literally?
Then check out the website "Where's George?" on which you can enter the serial numbers of bills and perhaps find out where they have been and eventually, as other bill-trackers randomly log in bills that pass through their hands, find out where at least some of them travel. Naturally, amidst the millions of bills that must be in circulation, and the relatively small number of people who will go to the site and log their currency, a "hit" in the form of a report back on one of your bills will not be common. But I've logged maybe 20 or 30 bills over a couple of years and have had one or two reports.
I'm not sure there is a lot of practical use for this information, unless the U.S. Treasury somehow finds the statistics useful, but it's an interesting curiosity.