Question

Why we need constitution

Answer 1

A legislature creates the laws. When a new legislature comes to power, it can repeal the old laws and create new ones. 
But most political scientists and philosophers believe that certain principles of the government be made less subject to the whims of the latest election. These principles form the foundation of the political establishment. Of course, they can be amended, but the process of amending these principle should be more difficult to accomplish.
 
Thus we have constitutions. In America, there is the Constitution that emerged out of Philadelphia in 1787. Each of the 50 states has its own constitution.   
Constitutions may enumerate certain rights that are reserved for a county's citizens: freedom of expression, of religion, freedom of assembly. Those charged with criminal offense may be entitled to due process including a trial by jury.
Interestingly enough, not all countries have constitutions.