The Founding Fathers thought that it was necessary to have a constitution because they believed in the idea of limited government.
The Founding Fathers believed that government should not be allowed to simply do whatever it wanted. Such a government would be able to tyrannize its people. With a constitution, by contrast, the powers of the government would be specified, as would the limits of those powers. Because these limits would be written, they would be known by all and the people could prevent the government from exceeding them.
Written constitutions set out the limits on a government, which is why the Founders wanted to have one for the US.
The key Founding Fathers, John Adams, James Monroe, Benjamin Franklin, Alexander Hamilton, Thomas Jefferson, James Madison and George Washington believed it was necessary to write the constitution in order to strengthen and further define the government as put forth in the Articles of Confederation.
Much of the constitution defines the various parts of the government and defines the roles of each. These checks and balances safeguard against any one facet of the government having too much power and taking away the voice of the people. The Founding Fathers wanted to be sure that a tyrannical leader could not take over the country.
They also knew that a constitution should provide details of the economic policies and taxes under which the country would operate. The constitution describes how the government pays its debts. It defines taxes and guards against the taxation without representation that the Founding Fathers experienced with England.
The Founding Fathers also felt it important to guard individual freedoms in the constitution. For this reason, they also addressed the freedoms each citizen would be guaranteed by the government.