1. Agents investigating Weiner saw evidence of State Department related emails on Weiner's laptop. This is a totally separate team of agents from the team that investigated the State Department emails. They thus have no idea whether these have already been reviewed or not. And they can't lawfully look at them without a search warrant or (this is key) consent of the owner of the emails. In theory, they could also send a grand jury subpoena for them to Huma and/or Weiner. Regardless, they would not be responsive to the Weiner sexting search warrant, so they can't seize and review them. They appropriately raise it up the chain.
2. The issue goes up the chain. Comey and his team decide they better get a look at these or they will be accused one day of a failure to follow a lead (they frankly should have had these long before). But they don't know what's on the emails, so they have no way of assessing whether they're relevant or not. They need a warrant, or consent, or a subpoena.
3. This is where I might be ready to give a slightly charitable explanation to Comey. He thinks if they seek a search warrant, it will leak. And that's probably a reasonable possibility given many GOP-friendly agents and lots of leakers apparently at HQ. In Comey's mind, headlines would be everywhere stating "FBI Seeks Search Warrant for New Clinton Emails" and the cable news folks would be in full hyperventilation mode. The FBI, his beloved institution, would then be subject to unrelenting attack for interfering in the election. So he decides he better preempt this by sending out a vague and, in his view, innocuous letter to the Hill. He thought he was in a no-win situation. One of his own making (he should have never commented on this case publicly at all), but still, a no-win. Chaffetz then distorts it and the uncritical mainstream news outlets desperate for a real horse race go berserk with hysteria and uninformed speculation.
But here's the thing: They never needed to get a warrant. All they would have had to do is ask Huma's and/or Weiner's lawyers whether they would consent to the search (or accept service of a subpoena for the emails). And they surely would have given it, knowing full well that the alternative would be the FBI getting another search warrant and that leaking to the press. It is not unusual to ask for consent to search from subjects, especially when you have possession of the data. And if they say yes, it's the most defensible way to search as the subject will effectively be waiving any 4th Amendment challenge to the search. If they had done this simple step, they would have the emails by now and would have been able to de-dupe them against the emails they already had and have any new ones reviewed in short order. By doing so, they would also have been taking steps to minimize potential leaks...
My take away: Nobody knows (yet) whether there is anything here or not. Comey released the extremely incomplete and prejudicial information against government policy, likely because he figured leaks from partisan agents would make him look bad.
So far, this is substantively a total nothing-bomb, but politically the press and Republicans are in heaven.