Through stories like the Satan myth, we can confront our own falliblity. If we don’t, then we condemn ourselves to the one true Hell: denialism.
Why do people believe these obviously wrong things? Many of them are stupid or sincerely troubled, but so are lots of people who never fuck up this bad.
Mainstream religions enjoy the privilege that they’re presumed to be authentic in some undefined way. But scratch the surface and the truth is not far down.