I think you misunderstand. A 16GB smartphone (a small one nowadays) is ENORMOUS. It can hold massive amounts of data, probably every email you’ve sent or received in the last few years; every text message since you got the phone; plus photos galore, taken by you or sent to you. Even if you only keep a week’s worth of email, it may have a tracking list of everyone you sent or received email from, to fill in your sending emails with the correct spelling of contacts. Plus, you may have saved attachments from your emails, even if the message itself is gone. I’m not sure how recoverable expired / deleted items are, but based on how PC’s work, it may be easy to recover(?).
You may have apps that track your path; but generally location data is data kept by the cellphone company. As long as your phone is powered on and ready to receive calls, even if the screen is off, it is pinging the nearest cell tower every few seconds or so to say “I’m still here” so the system knows where to send incoming calls. For some reason, the cell company stores that information for a while (I think). Certainly, in real time it would be trivial to track. Since most cells include a GPS nowadays, the tower can also ask that data in order to enhance cell signal reception and hand-off to the next tower.
The FBI, as I understand, wants to see if there are any extra email accounts on the phone, and if so, who the emails were to/from. Considering the couple specifically destroyed their personal phones before going postal on his workplace, but left this one intact, odds are there’s nothing worthwhile on it.
But once Apple produces the software to crack this phone, there are hundreds of phones help by police all over the country awaiting the same treatment for less high-profile, less compelling cases; so Apple knows this is the thin edge of the wedge. Samsung couldn’t have planned it better to trash American tech’s reputation if they had distributed free iPhones themselves to terrorists.