An amendment is a written change, an addition to an earlier rule or idea, which begs the question: why was there a need to amend the Constitution only two years after it was ratified?
The anti-federalists had always wanted a specific Bill of Rights in the original document, but by 1789 they had already been deliberating for almost two years, and there was the distinct possibility that the convention could break up without agreement on any document at all. Through considerable lobbying by key Federalists, they were able to convince Jefferson and Madison to wait on the Bill of Rights until after ratification, then Washington and Franklin helped them push it through Congress and the states by 1791.
The first ten amendments to the Constitution, better known as the Bill of Rights, were added to the Constitution to allay the fears of opponents of the Constitution, commonly known as Anti-Federalists.
The Anti-Federalists feared that the national government, if given more power the way the Constitution proposed, would abuse the rights of the people.
The Federalists, who favored the Constitution, needed Anti-Federalist votes to ensure that the Constitution would get ratified. So they added the Bill of Rights to guarantee that the new stronger federal government would not be able to trample people's rights.
The first 10 amendments of the Constitution are called the Bill of Rights. They were added to assuage Ant-Federalist fears that a strong federal government will tramp on the rights of the people. Federalists would argue that rights don't need to be explicitly given or stated in this manner on the very definition that they are, in fact, rights.
The anti federalists needed the bill as a form of assurance for the Constitution to be ratified lest their opponents abuse the rights of people.
Definition of constitution: a body of fundamental principles according to which a state is governed. In this case, it is to address the concern that the rights of the people to be abused.