Holding a belief that cannot be reconciled with reality can be harmless if it can stay isolated, but in practice that doesn't always seem to be the case.
What ends up happening is that these things sooner or later lead to an internal conflict. To fix this, a healthy person realizes that their belief is silly, and over time adjusts to realign more with reality again. This doesn't have to be a conscious process, but it helps to be self aware and open-minded.
In a different group of people, what happens is that they double down. Their belief X conflicts with reality Y, so a new explanation Z is needed. Z also conflicts with reality, so the pile grows. What makes is worse is that the internet created spaces for these people to find each other and provide increasingly absurd extensions to their twisted view of the world.
Someone who starts out thinking the election was rigged may find themselves in a group that claims COVID was part of the election fraud plot, that it will go away when the new president is sworn in. Then it doesn't, so a new explanation is needed: The idea of microchipping/depopulation/etc. solidifies. Someone needs to be blamed and there happens to be a leftover grudge from the elections, and look at them promoting masks and the jab! It's obvious, they're not even hiding it anymore. And on and on it goes. Eventually we arrive at "there are no coincidences" and "symbolism will be their downfall", where Twitter timestamps are somehow proof of a hidden agenda, and words mean whatever the listener wants them to mean.