Thanks to Hans for reminding me about one use and pointing out another.
Simply put – keep them, or at least your most recent ones.
Countries such as Tunisia insist on proof that you’ve disposed of all (or most of) their currency when you leave. As such, they want to see how much you’ve withdrawn while you’re there so they can work out if you could reasonably have spent it.
In India, if you’re buying train tickets with cash, you need to prove how you got hold of it (or something). Whatever the reason, staff will insist on seeing ATM slips that cover the amount you’re spending or they won’t sell you the ticket. Of course, you can always book them online on credit card if you can register with the relevant web site.