They offer simple answers to the BIG life questions.
And the prospect of (certain forms of) love and friendship to awkward folks, as long as you follow the rules. This can be really powerful for people who have difficulty making friends on their own.
Sometimes, people know that their understanding of reality is suspect. So when some unapologetic-ally prominent assertions are confidently made, all of a sudden the sense of responsibility shifts: from the person to the group. The freedom to be wrong can be terrifying.