Which is what?
If you don't know the reason then you have proven my point that atheists have less than impressive intelligence.
You're the one who said there was a good reason, so what is it?
That it provides a functional advantage.
Not any more it doesn't. Theists today are mostly nuts, Muslims, Jews, Christians (especially in the US)... The scientists and other smarter people are moving away from those backwards concepts.
I would surmise that the 'smarter people' have never been all in from the beginning. Religion acts as societal glue. The more intelligent play the game because of the social ramifications of not doing so. The many laws and precepts are merely social codes that should be adhered to for the good of the society. The intelligent, tend toward some level of agency. In order for these more intelligent elements to get anything done they need a control mechanism, extra-human law. IOW, they need a way to corral and control the dimmer elements of their society. Religion and it's "Big Boss in the Sky/Karmic Law" principle is exactly what an orderly society requires. Especially if that society is populated with <90 I.Q. mouth breathers.
In this way religion is very important and good. Without it, there is lack of direction and subsequent disorder and social decay....as we are seeing in modern culture.
Now, of course I am speaking in the traditional, simplistic, Ten Commandment sense of religion. All things, even religions tend to corrupt with time.