So what is the reason?
To try and pop this balloon and nullify it in the American mind.
If Trump had NOT repaid Cohen, then Cohen could be in jeopardy for making a "gift" to Trump without paying gift taxes. Also, given the timing, the Feds could construe this as an "in kind" campaign contribution, which could also screw Cohen.
So, Trump did the proper thing... he reimbursed Cohen for expenses made on his behalf. Rudy simply set the record straight on that, thus relieving Cohen of any concern over the issue.