We need a constitution to govern a country properly.
The constitution defines the nature of the political system of a country.
Sometimes we feel strongly about an issue that might go against our larger interests and the constitution helps us guard against this.
All the three organs of government (Executive, Legislature, and Judiciary) functions within the constitution.
All the three organs of govt. including ordinary citizens, derive their power and authority (i.e Fundamental Rights) from the constitution.
If they act against it, it is unconstitutional and unlawful.
So, the constitution is required to have an authoritative allocation of power and function and also to restrict them within its limit.
We need a constitution to perform the following functions:
1. To specify who has the power to make decisions in a society. It decides how the govt. will be constituted.
2. To set some limits on what a govt. can impose on its citizens. These limits are fundamental in the sense that govt. makes sure that citizens may never trespass them.
3. To enable the govt. to fulfill the separations of society and create conditions for a just society.