^ some pokemon can only be obtained through nintendo events or codes, like celebi, mew, deoxys, jirachi, and others. If you don't go to the nintendo event, which is very rare and hard to get to, you have to obtain it code-wise. =/
also, say you have sapphire, but you don't have ruby or any other game. You can't catch all the pokemon in the game, because you need to obtain some pokemon from the other games by trading. You can use codes to obtain the pokemon you couldn't get =/