The Bill of Rights was added to the U.S. Constitution primarily to calm the fears that the new federal government would violate states' rights or people's rights.
Many of the founding fathers and numerous citizens were concerned that the Constitution did not adequately protect individual liberties and rights against potential government infringement. The addition of the Bill of Rights, which comprises the first ten amendments to the Constitution, was seen as necessary to ensure the protection of fundamental rights such as freedom of speech, religion, and assembly, as well as protections against unreasonable searches and seizures, among others. This compromise played a significant role in the ratification process of the Constitution, particularly appeasing those who feared centralized federal power.
While the Bill of Rights does address individual rights in relation to the federal government, its primary purpose was to establish clear protections to alleviate broader concerns regarding the potential overreach of government authority.