Whether they like them in the real world or not, when it comes to their own worlds, plot creators—writers, GMs, you name it—tend to love rumors. Why?
Rumors are everywhere. You don’t have to worry about justifying people maintaining such a perfect oral tradition that the one little poem that perfectly describes how to circumvent the latest problem is one of the most popular children’s songs, or make absolutely sure that your character(s) can come in contact with the prophecy you need but that the source hasn’t figured out how to untangle it yet; they can as easily be about something minor like a new person in an old neighborhood or the possible infidelity of one of the locals as about something major like what really happened to the missing princess or what the secret weakness of the big scary antagonistic thing is—if anything, they’re likelier to be about the little things than about the big ones. And best of all, rumors come from a love of gossip, and a love of gossip is practically hardwired into humanity.
Rumors are easy to mask. Part of it is because nobody’s entirely sure what’s going on—if they know exactly what was happening, it would be a fact, not a rumor. Part is that by the time anyone with narrative importance (or at least, anyone with narrative importance who wasn’t responsible for seeding them in the first place) hears them, they’ve been warped and confused and misheard—we can’t really expect something that spreads by Telephone to be spread without alteration, now, can we? Part is that enough of the things go flying around a given setting that it’s easy to camouflage them: we can get the rumor in the air, but maintain suspense or ensure the character needs to work for them by embedding them in a mess of other things that the locals would be interested in.
Rumors give us plenty of opportunity to give the people relaying them and the culture around them a little bit of their own voice, giving us a little extra free incidental realism. Which rumors are going around, for instance, says something about what sorts of things the people spreading them prioritize, and what the culture around them encourages them to (or at least doesn’t discourage them from) discuss—are they talking about their peers’ offspring? About mysterious events that have been going on recently? About potential scandals? Badly behaved pets? What such and such a famous personage was wearing for a major event? Delivering them generally involves slipping into voice; there’s a lot that can be done just by choosing slang and figurative language to get across a sense of what the character sees.
In short, the rumor is a plot Swiss army knife that requires only a small group of people who are both somewhat inquisitive and willing to talk to each other in front of the character that needs to hear it. Why wouldn’t a plot creator love a few good rumors?