My guess - and consider it only a guess - is that the FBI is treating this new cache of emails to be relevant only because HRC was either a recipient or sender, and therefore they need to be evaluated for relevancy.

Odds are that this will be a big nothingburger, because in the 30k+ emails previously reviewed, there was no criminality despite the FBI’s own awful handling of the matter (not getting that (c) in the wrong place means that the document wasn’t properly classified for instance).