In confederate systems, the policies of the states and provinces keep full power over their internal affairs. What is the purpose of this arrangement?

You have to remember, I think, that the Federal government has full power over certain things like the ability to raise money to go to war. The bill of rights for the most part, is enforced by the Federal Government, but it is also interpreted by the states or provinces.

It's complicated.

The main reason for giving the states power is that you are getting closer to who does all the electing (the people). They should in a democracy like America, have a great deal to say about how they are to be governed.