From a publicity point of view it's golden, doesn't cost anything and can increase exposure.
Why people care is what I don't quite get, but I've never really entirely understood why some things become popular anyway.
A key point here is it was wildly inappropriate for Comey to recommend no prosecution in this case on TV. It is totally not his decision. The prosecutors in the DOJ are the ones who get to decide if prosecution is warranted. The FBI's job was to investigate and generate a report to the DOJ. They do get to make a recommendation regarding prosecution but it is only a recommendation. Comey absolutely should not have announced the recommendation at a press conference before the DOJ has even started reviewing the final FBI report. It reeks of prejudicing the entire case since it places inappropriate pressure on the prosecutor in the DOJ to not prosecute when they may well be inclined to prosecute when they see all the evidence.
Comey s assertion that Clinton and her people had no intent to do harm by mishandling top secret compartmentalized information so they should not be prosecuted is also way over the line. The fact is they did mishandle top secret information, and it is unknowable if that mishandling resulted in the information being accessed by foreign powers or others who were not authorized to see it. You knowingly mishandle classified information in violation of the oath you signed there have to be consequences otherwise why should anyone bother to protect classified information. If Clinton is elected President how can she expect the millions of Federal employees working for her to protect classified information when she knowingly didn't and got away with it.
Thirdly mishandling email is only part of the case against the Clinton. A key reason Clinton may have been using this private server is there may have been email between her, foreign governments and affluent individuals who were donating large sums of money to the Clinton Foundation while she was Secretary of State creating the appearance that she was soliciting bribes in return for favorable decisions from the State department on things like arms deals. Clinton is claiming these are personal emails so she withheld them from the FBI but they may be a trail pointing to public corruption.
It smacks of whitewash to suddenly short circuit these investigations so Clinton will have a clean path to the nomination at the convention which is just a few days away now.