Comment by Grustaf

The most important effect of all their rules is of course that it creates a cohesive community. If you look at it as a group evolutionary strategy it’s incredibly successful.

And of course most rules and laws in society are “arbitrary”, but in order to be applicable on a mass scale they need to be measurable. For example, there is nothing magical about the age of 18 or 21, but there has to be some easily measurable rule to prevent young and immature people from drinking. So we choose a completely “arbitrary” number like 18 and stick to it.