Hello there!
The purpose of The Bill Of Rights is-
A list of the most important rights to citizens or inhabitants of a country. The purpose is to protect these rights, and the bill of rights reinforces them.
For ex)
If someone were to disobey a law or principle, the only way to know is to have the law written down so everyone knows it (adults). Not writing something down, or not making it clear, people will not know how to react when they are charged for something they did not know was illegal.
Hope this helps you!
~DL