The Declaration of Independence states the purpose of our government is to protect citizen’s God given natural rights to Life, Liberty and Property (changed to Pursuit of Happiness). The Constitution is the document that provides the rules to accomplish this task.
For example, one principle implemented by the Constitution is a concept the founders called Federalism, by Article 1, Section 8 and Amendments 9 and 10. These rules explicitly define the role of the federal government with respect to the states and the citizens. This prevents the federal government from violating citizens natural rights, and it has many other advantages, e.g., only the federal government can print money, making it easier for it to overspend, and to deflate our money by printing too much of it. States are forced to balance their budgets. If the federal government had never involved itself with Food Stamps, Welfare, Social Security, Health Care, etc., we would not be in the financial death spiral are now in.