Proofs also teach the discipline of having a supporting reason for every statement.
I too have to agree with this. As with many other math concepts, some kids automatically have it and others have to see it step by step. Another thought is that if the student does not put down each and every step in the sequence, there is no way to prove if he got the correct answer in the correct way. He totally misapplied the concepts he was using. If he had only needed to put down the answer, and if that was all I looked at, I would not know where he went wrong.