The harm is in stochastic terrorism. You can make up lies that are intended to spur people who feel they have nothing to lose into action.
https://csl.mpg.de/en/projects/philosophical-and-public-secu...).
It's similar to the concept of blood libel: https://en.wikipedia.org/wiki/Blood_libel
For example, imagine that online discourse on some fringe website that has a few hundred thousand members devolves into a theory that some house in your neighborhood is harboring a secret dungeon full of children underneath it. There is no direct threat to you per se, but a few hundred thousand people think that a house - maybe your house - has a basement full of unspeakable horrors. As the rumor spreads and grows the chances of your house being broken into by one of the believers goes up exponentially. Suddenly nobody in your neighborhood is safe anymore as random weirdos keep scoping out your houses and following you to and from the grocery to see if you're the one with the dungeon beneath your house. Eventually one of these weirdos pulls a gun on your neighbor and demands to see his basement and while there was no dungeon the firearm accidentally went off and killed your neighbor. Your neighbor's death would trace directly back to the spread of this rumor.