Why do we need a constitution?

To protect the citizenry of the country.

The reason a Constitution is necessary is very simple.  Its main goal is exactly what Ancient Hippy said.  It protects citizens of a country(In the most relevant case, the United States) from tyrannical government overreach.  To fully understand the need for such a document, one must look to history and how other governments treated their citizens.  In almost every other governmental system in history, a central power, class, monarch, etc. Has had sole power.  

The Founding Fathers of the United States studied these governmental structures and came up with the system of government that we have today, a Constitutional Republic.  The constitution lays out the structure and function of the Federal Government, as well as lay out issues that fall outside the jurisdiction of the Federal Government.  Politicians and "Constitutional Scholars" will make the argument that the Constitution is a document of "negative liberties", therefore it needs to be changed...

While their initial analysis is correct, the action is incorrect.  It is a document of negative liberties in the sense that it states what the government CANNOT do, which grants the individuals of the nation the liberty to fully realize their inalienable rights: Life, Liberty, and the pursuit of Happiness.  There is so much more that can be said about our Constitution.  I would highly recommend reading the Constitution in its entirety, then look at different governmental systems, and compare the pros and cons of each.  Which one serves the individual, and which one is served BY the individual.  Hope this will give you a better understanding.

God Bless,

Michael