A written constitution clearly defines the division of powers within a government, the powers and limitations of each branch therein, and the rights of the citizens at large. Written constitutions also allow literate citizens the opportunity to familiarize themselves with their government's laws and infrastructure, increasing the power of individuals.
The function of any written constitution is to express guiding principles for the state or nation, establish fundamental rules, and outline the maximum and minimum power of the state or nation. Since 1789, national constitutions have lasted an average of 17 years before being replaced. The Constitution of the United States of America is an outlier. It has been the primary legal document of the United States for over 200 years.