Why did nations build constitutions and laws?

The answer is:

A constitution is a set of fundamental principles or established precedents regard to which a nation or other system is govered.

Explanation:

Constitutional law deals with the fundamental principles by which the political exercises its power. In some instances, these principles grant limited powers to the government, such as the power to tax and spend for the welfare of the inhabitant.

It is the most authentic political legal written document in which the political system of the nation, formation of the organs of the nation and use of the power of the nation are mentioned. It determines the right of people and power of the government.