
Sometimes it is not enough for software to perform correctly. You also want the implementation to be clean. Most software spends many years being maintained. It is not success if you hack together something that will cause a disaster later.
The goal of code/peer review is to improve the quality of the code. This may involve detecting subtle bugs. It may also mean that the code itself is improved. Once you get to that level, you have a better chance of producing that high quality code.