First, in what has been a relatively news-starved week, this is big news whether there's anything to it or not.
Second, what is the "unrelated case" in which these new emails arose? We don't know anything directly. But we do know that the FBI recently opened an investigation into the theft and dissemination of John Podesta's emails. That seems like a pretty good candidate for the investigation that might have generated new emails. (As I wrote this, CNN reported that the new emails were not tied to Wikileaks or the Clinton Foundation. So perhaps not. We'll see.)
Third, I would recommend everyone look at what Director Comey actually said. It's far more couched and limited than the way it's currently playing in the politics and news world. The letter says investigators "learned of the existence of emails that appear to be pertinent to the investigation. I am writing to inform you that the investigative team briefed me on this yesterday, and I agreed that the FBI should take appropriate investigative steps designed to allow investigators to review these emails to determine whether they contain classified information, as well as to assess their important to the investigation."
We should note that the existence of a small number of emails with classified information that passed through Secretary Clinton's server was not the factual matter that determined the FBI's decision not to recommend prosecution. That fell on her intent, behavior in setting up the server, and what the FBI determined was her truthful answers during the inquiry.
From what Comey said in his earlier comments recommending against prosecution, there was little reason to think that additional emails with classified information would have changed the outcome. That just wasn't the basis of the decision. And determining whether there is any additional classified information on these newly uncovered emails seems to be the main focus of the review.
What could change this calculus is if there were new information about the intent behind setting up the private server. Something like that could change the equation. But that seems pretty hypothetical. And again, the primary focus in Comey's letter was ascertaining whether there was more classified information.
Fourth and finally, Comey did not say he was "re-opening the investigation". Rep. Chaffetz (R-UT) said that. It is to some degree a matter of semantics. The investigation was concluded and closed. Now there's some new evidence they're going to review. By some definition that is 're-opening'. But obviously the wording is far more explosive, as Chaffetz of course realized. Chaffetz defined the initial coverage because he had the first bite at describing what was happening.
We all know that the substance isn't the same as the politics. But the shockwave set off by this announcement and the headlines about 're-opening' the investigation seem far, far out ahead of what Comey's letter actually said. The significance of the letter (to the extent we can glean it, which is limited), based on Comey's earlier explanation of the investigation, seems likely to be limited and procedural.
As I've been writing this post, a number of news organizations that were reporting a half hour ago that the investigation had been 'reopened' are now saying that's not the case.