Thomas Jefferson, in his wisdom, wrote “In questions of power, let no more be heard of confidence in man, but bind him down from mischief by the chains of the constitution.”
The dictionary offers a drier take on the role of a constitution: “the basic principles and laws of a nation, state, or social group that determine the powers and duties of the government and guarantee certain rights to the people in it.”
Whether you prefer the vivid imagery of Jefferson or the academic definition, the point stands that a constitution exists to preserve liberty by restraining the government. A proper constitution, put in place by the governed, sets hard limits on the powers of those who govern, binding them down from mischief and overreach.