I'm pretty sure BBS is to explain how the Heartless were formed (since you fight the Unversed, new enemies, no heartless or Nobodies methinks), why Xemnas went evil, etc etc. Basically to explain the keyblades and the backstory to further emphasize things in the other four. Also, Kingdom Hearts: Coded does have relevance to the continuity of the series; look up the ending. It's on YouTube somewhere.