It gives meaning to a persons otherwise pointless existence, a way to control people by instilling fear of the afterlife.
The fact that all religions are basically the same set of ethics and morals, yet, they all claim to be the right ones, and everybody else is wrong.
People thrive structure as it implies a sense of fairness rather than a survival of the strongest. Most religions cater to the poor, uneducated that are prone to be treated unfairly.

I guess one can compare Organized Relgion to Organized crime. Instead of protection from the other guys, you get protection from the devil, then you owe them a little debt after.