Why is it necessary for a …

It is most necessary for a country to have a clear demarcation of powers and responsibilities in the constitution. Powers of all institutions like the legislature, the executive and the judiciary and even independent statutory bodies like the Election Commission are demarcated.
It is to ensure that no single institution acquires monopoly of power. This ensures that even if one institution wants to subvert the constitution, others can check its transgressions.
For example, in Indian Constitution the principle of check and balance is applied to secure the fundamental rights to its citizens. In the absence of such a demarcation there would be problems to the citizens and the laws will be unjust and unfair. A constitution is to specify who has the power to make decisions in a society.
It decides how the government will be constituted. In a democratic constitution, it sets some limits on what a government can impose on its citizens. These limits are fundamental in the sense that government may never tresspass them.
The constitution shows how the organs of government should relate to each other. In a federal form of government, the judiciary is given a specific place. It can declare any law unconstitutional or ultra vires if it is not in the tune with the provisions of the constitution. If there was no demarcation of powers in centreand states, there would be a crisis and the federal set up would get strained.