Another big mistake is to imagine that just knowing what to accomplish is sufficient. It's not.
Maybe if you really, truly, could know what to build, get it perfectly right the first time, and never have to change it again, quality wouldn't matter.
Even oracular powers won't prevent you from having to change your code, though, because shifting market conditions mean that what you need tomorrow isn't necessarily the same as what you'll need a year from tomorrow.
The ability to change code quality is valuable on its own, for that very reason. The fact that it mitigates our inability to guess what people want or what will happen in the future is just an extremely useful side-effect.
