Compare the Mississippi black codes, found in document 15-3, with the Civil Rights Act of 1866, found in document 15-4. Why do you think Congress believed that it had to pass the Civil Rights Act and then adopt the Fourteenth Amendment?

Since there were no documents attached, I have located a copy of the black codes in Mississippi. I am using that source along with the source for the Civil Rights Act of 1866 to respond to your question.
The black codes of Mississippi, and for that matter other states, attempted to strip African-Americans of some of the freedoms they gained after the Civil War ended. For example, these laws prohibited most former slaves from carrying weapons. Additionally, former slaves could be fined if they didn’t have a job.
These black codes convinced some people that there had to be some guarantees of the rights of the freedman. The Civil Rights Act of 1866 gave full citizenship to African-Americans. The federal government would protect these rights. When President Johnson vetoed this bill, Congress overrode the veto. This convinced the Radical Republicans that a constitutional amendment was needed to guarantee the freedoms of the former slaves. While a law can be reversed at any time, it is much more difficult to change the Constitution. Thus, if an amendment to the Constitution was made guaranteeing freedoms for all citizens, including the former slaves, it would be much more difficult to take these freedoms away.
https://www.blackpast.org/african-american-history/1866-mississippi-black-codes/