Rationale
n“Point out needed improvements in a product of a single person or team.” n“Confirm those parts of a product in which improvement is either not desired or not needed.” n“Achieve technical work of more uniform, or at least more predictable, quality than can be achieved without reviews, in order to make technical work more manageable.”
The authors of this red book, Freedman and Weinberg, list some reasons that code should be reviewed.

On the one hand, the review can point out needed improvements in a product.  This probably happens a lot.

On the other hand, it can identify places where improvement is not needed.  I can say that I not too infrequently spent too much time perfecting something, often a user interface, when it is not really necessary.

The last reason here is related to process improvements.  The work comes out more predictable as more people have contributed.