For playing Final Fantasy XIII, I received a Key Item called "Eternal Crystal". Most Key Items don't really do anything outside of reminding you what you have for purposes of the plot. Does this one do anything special sometime though since it's optional?
The Eternal Crystal doesn't do anything: it's just there for bragging rights. If you play on the PS3, you also got an extra theme for playing Final Fantasy XIII.
Most key items do have a purpose, though:
- Artefacts open up new gates
- Maps unlock the maps for the area you're in
- Gate seals let you close gates and replay areas from the beginning
- The vast majority of miscellaneous key items are actually side quest objectives
The Eternal Crystal actually increases your luck at Serendipity's slot machines.
Here's the link to IGN (I believe). It clearly states that the Eternal Crystal adds luck to the Serendipity Slot Machines http://www.ign.com/wikis/final-fantasy-xiii-2/Special_Unlocks