Why did congress pass the civil rights act of 1866?

All persons born in America were citizens, without regard to race, color, or previous condition. 

It was the beginning of the new era. It purposed that all persons born in the United States would now bcome official U.S citizens without regard to race, color, or previous condition. As long if you were born in the U.S, you had rights to be called a U.S citizen. Why congress did it was because of slavery and people calling themselves citezens when they were not