The FBI has already reviewed tens of thousands of e-mails, and hasn't found any criminal activity in them.  There is no reason to believe that these e-mails, which for all we know are duplicates of ones they have already reviewed will show anything different.  Yet the Trump camp are trumpeting them as some sort of smoking gun, even though they have no idea what they contain.  The reason that Comey's action was illegal is that it allows Trump to smear Clinton with completely unfounded speculation and innuendo days before the election when she isn't able to access the e-mails to defend herself.  It may or may not have been Comey's intention to provide the Trump camp with this blunt but hollow weapon to pummel Clinton with, but it took only milliseconds for the Trump campaign to take it up and wield it, as insubstantial as it is.  This is the whole reason that the Justice department prohibits this type of disclosure in the last weeks of an election.  Comey should have known better.