Purpose unclear. Why are the FBI *really* trying to subvert encryption?

Tim cook says Apple will fight a federal order to help the FBI hack an iPhone.  

An earlier version of this page has a paragraph which seems to have been deleted later;

It was not immediately clear what investigators believed they might find on Farook's work phone or why the information would not be available from third-party service providers, such as Google or Facebook, though investigators think the device may hold clues about whom the couple communicated with and where they might have travelled.

Is that "Whom" grammatically correct?

This does raise a 'why' in my mind.
Cant the other service providers (who would it be, AT&T, Verizon?) supply the 'traffic analysis of who they communicated with? Isn't this the sort of "metadata" that the government spies are supposed to be collecting?

Opening the phone won't give the content of the messages past, they are gone like the snows of yesteryear[1]. Dead as the author of that famous quote.

So what are the FBI looking for? The address book? I'm not sure how helpful that will be and its likely to cast suspicion on innocent parties.