A bit of everything, I suppose. Test case for the legal side / setting a precedent. Test case for the technical side. Test case for 'can we make Apple play ball?' Unwillingness to disclose full abilities (and yes, this works both ways) of TLAs involved. Getting maybe data that is admittable in court (but that's rather a front than a reason - the perps are dead and so are their private phones. Sure, it's nice to have all the data to wrap things up and have a file without any loose ends, but it's not like a conviction hangs on this).