Well, obviously, you're right that we don't know what's in this batch of emails. Judging by the number reported, I suspect a whole, whole lot of them will be spam.
I think the supposition about there being something significant in them stems from the FBI's surprise declaration. If they were perusing the messages and came across nothing but Nigerian phishing scams, massive discounts on Viagra, and the occasional pic of Anthony's Weiner, then I can't imagine they'd have done what they did.
Despite all the flak he's taking right now (and took several months ago), James Comey has a reputation of doing things by the book and without regard for the underlying politics. So I'd say it's natural to guess that they came across something significant, given his lurch back into the political crossfire 10 days before an election.