Image: Naughty Dog / PlayStationOfficial screenshot from Naughty Dog / PlayStation.
It Can't Be for Nothing is the platinum trophy in The Last of Us Part I. It unlocks automatically once you hold every other trophy on the list, so it marks a full completion of the main story and Left Behind.
Finish both campaigns, then clean up the collection sets (pendants, comics, manuals, notes and artifacts, conversations, workbenches, safes, tools and shiv doors) and the one-off moments such as petting Buckley or winning the brick contest in Left Behind.