What is the purpose of the rules in The Giver?

There are many different rules in the society depicted in The Giver. But their purpose is the same as in any society, including our own: to maintain order.
Without rules, however they are defined, society will collapse. This is especially true of authoritarian societies, such as the one we see in The Giver. The elders of the community have established very clear policy objectives; they have a very specific blueprint in mind for what kind of society they want to build. And if they're to build that society, they need to have in place an exact set of rules that will help them to achieve their overriding goals.
As the elders have deemed absolute conformity to be the order of the day, they need to have rules to ensure that such conformity is maintained, come what may. Otherwise, people would almost certainly embrace their natural differences, thus undermining the community's governing principle of Sameness. Without rules to control them, people would spontaneously assert their individuality, with catastrophic consequences for the elders of the community and the gigantic project of social engineering they've painstakingly put together over many years.