Codules would basically let you build your own commands, and the smaller the list of DNA commands the easier it is to test and debug.
For evolved code, the smaller the building blocks for evolution to play with the more impressive the final results are. Codules, again, let bots build up code reuse so it's entirely possible for things like storeadd to evolve on their own just by happenstance. Or other, weirder DNA shortcuts.
Or that's the thinking anyway. I haven't played with it in a long while; I'll revisit it at some point and finish flushing out the language once I have a chance to play with it in practice.