If a character in almost any type of fiction gets a chance to eavesdrop, they will almost certainly hear something important. So, for instance, if our hero finds himself hiding in the villain's base listening to a conversation, chances are it's going to be about the villain's latest plans for world domination, rather than two random Mooks talking about their weekend plans. It's also very likely that the villain will take this opportunity to go over important information everyone they're talking to should already know, in order to give the hero all the context and backstory he needs. In this way, this trope can function as a sort of in-story version of As You Know, only instead of filling in backstory for the audience, it's being done for the character, and is a way to get vital, plot-movin