Breaking up the party helps us identify the characters as individuals rather than as just parts of a whole.
In Final Fantasy XIII, most of the cast is introduced in the very beginning. Rather than spread these introductions out over the course of the game like other RPGs, the cast comes together after just a few hours and then breaks up again. It’s a strange series of moments, seeing your party systematically disbanded, but the reasons behind these divisions are very personal, and as we watch each character go their separate ways, we learn a lot about their inner thoughts and desires.
First some background information on the world of Final Fantasy XIII: the game takes place in Cocoon, a protective sphere separating humanity from the dangerous outer world of Pulse. The fal’Cie are powerful magical beings that reside in Pulse. They attacked Cocoon a thousand years ago and were repelled, but the attack left humanity paranoid and forever fearful of these creatures. Adding to these fears is the ability of the fal’Cie to turn humans into servants, the l’Cie. When branded as l’Cie, you’re given a vague vision of a task that you must complete. If you’re successful, you turn into crystal; fail, and you become a monster. Either way it’s a death sentence, and one more reason to fear the fal’Cie. Yet ironically, Cocoon was built and is still maintained by these magical beings. They control everything from day and night cycles to weather patterns.