A trainer is an external hack program for the game, so unlike the developer-created cheat codes which the game recognizes as cheats, trainer cheating can potentially go undetected by the game so metaverse cheating might be possible with one. (IIRC, If you use the built-in cheat codes in a game, it is no longer eligible for metaverse submission.)
I'm all for having cheat codes in games, for the single-player to use to experiement and have fun with. But when game developers don't provide built-in cheats, trainers are a great way for single players to still do that. That said, though, with the many built-in cheat codes in GC2, there's very little reason to need a trainer besides being able to use it where you *couldn't* use the cheat codes (ie: a metaverse game).