There is an official definition in the Glossary section of the
Rules:
In Play
Hero cards, all face-up cards in each Hero's stash and hoard, and all Mischief cards and hired Monsters in the DMZ are in play.
As for use cases, players will often double- or triple-up on cards that are central to their strategy. If those cards are things that remain in play, like monsters, loot, or curses, this can penalize them while they try to make use of their planned strategy. (For example, I used multiple copies of the Groupies monster in my Bard's Final Countdown deck.) Also useful if your local players all prefer certain cards that you may also have in play, as this card does not specify that it only applies to their cards.