[Xhad]

Not mine, other people's that have folded. Sometimes you pick up a backdoor fd or something on a later street and it's helpful to know that three other people were showing cards in "your" suit on sixth street. Or, sometimes you know someone is unlikely to have trips when he bets after his board shows 22 because someone else pitched a 2. In the extreme, sometimes you can tell your hand is the literal nuts only because the only cards that make a hand better than yours are spread out between multiple hands including one that was folded on fifth street, where without that knowledge it would be incorrect to keep raising.

EDIT: I'm talking just about the stud games of course.