4 Answers

Jareds story...http://www.jaredstory.com/the_constitution.html

~We~ don't require every country to have a constitution, but we have one in the USA, other countries have their own constitutions Magna Carta or similar policies that keep the government under control.  Without such an agreement, we would have political anarchy, Constitution in the USA is to protect the people from the g'ment. 

You must have a draft or a blueprint before you build anything big.  The constitution is the blueprint of the government. As time marches on, it needs to be added to or addendum - amendments - These amendments are very difficult to  pass. Not everyone in the house agrees with these changes.  Right now there are many changes to the US constitution that is being discussed. IMO, I hope none of them make it through. 

A constitution is meant to protect citizens from a government overstepping its authority. But if you read up on what's happening in the US, EU, Australia and other "western" nations the governments are undermining these constitutions in favour of company profits! 

So everybody lives and conducts life by certain rules otherwise the country will be unruly and chaotic.