There was not enough evidence that Johnson was guilty of "high crimes and misdemeanors." Johnson was supposed to be a rubber stamp for the Radical Republicans in Congress. He was expected to tow the party line and make the South suffer for the Civil War. When Johnson would not, his own party attempted to take him out.
It gave other races the knowledge to know what is right and what is wrong. It was a defining moment in interracial empowerment. It gave the minorities a chance to fight for their freedom.
The answer for this question is a