To have been nominated by president Trump for Deputy Attorney General in his Justice Department, Rod Rosenstein must have been in agreement with Team Trump that James Comey was a problem, that Comey was instrumental in obstructing justice against Hillary, so considering that Trump could have fired Comey without a letter from Rosenstein’s citing the many reasons it would be appropriate, that Trump did request the letter shows that the president was anticipating Rosenstein’s opening of a criminal investigation of Trump for having fired Comey.
