To outline the rights of individuals.
When the constitution was written the individual colonies were asked to ratify it and so give up their independence and form the United States. Since a war for independence had just been fought, the colonies had concerns that the new federal government could become tyrannical and take away the freedoms they had fought so hard to win.
The Bill of Rights was added to the constitution to make it more palatable and remove some of the fears about losing states rights. Even the states that were willing to ratify the constitution without the rights listed were happy to see them spelled out .