why is it necessary for a …

One of the most important functions of a constitution is to set some limits on the rulers. These limits are fundamental in the sense that the rulers may never trespass them. Constitutions limit the power of the government in many ways. The most common way of limiting the power of government is to specify certain fundamental rights. Citizens will normally have some basic rights of liberties: the right to freedom of speech, freedom of conscience, freedom of association, freedom to conduct a trade or business, etc. These freedoms of the people could not be checked by the government. But during the national emergency or in national interest these rights may be withdrawn. The rulers or the government can impose some limitations on its citizens. There can’t be any constitution that gives no power to its people. In a monarchical constitution, a monarch decides but in democratic constitutions, the people get to decide. The public mandate is necessary to enact the policies for the rulers. In a dictatorship also the ruler has to get the support of the people to cling to the power as in the case of Pakistan, where General Musharraf conducted periodic ‘referendum’ to justify his hold on power. So it is not possible for a constitution to exist that gives no power at all to the citizens.