[WillDaSilva]

The harm people cause by holding believing false beliefs is rather complicated at times, yet extremely important and impactful nonetheless.

For flat earthers in particular this video sheds some light on why their way of thinking is harmful: https://www.youtube.com/watch?v=JTfhYyTuT44

To be extremely reductive, allowing false beliefs into one's web of beliefs corrupts it. This typically gives rise to a multitude of other false beliefs. Flat earthers almost never only believe one obviously false conspiracy theory. They become epistemically susceptible, and worse still, they tend to spread these awful ways of thinking with great zeal.

In the more general case, I recommend you learn about the ethics of belief. The Standford Encyclopedia of Philosophy page on the topic is a good starting point: https://plato.stanford.edu/entries/ethics-belief/

Maybe someone can come along and answer your question in a more succinct yet equally (or ideally more) convincing way. This request that you commit several hours to educating yourself is the best I can do at this time.

[deepnotderp]

Well said. I propose therefore that we ban Boltzmann for his crime of false belief in statistical mechanics. /s