During code reviews, developers have to explain the code they wrote to the team, most times, the review is centered on why a particular code was written. Usually, developers end up justifying why they used a particular style or method to engage a challenge as against other alternative methods. Code reviews are meant to be done frequently in a project because it is very possible for a developer to forget the code written in a particular project and why that method was chosen (if there is no documentation for the code).
While it seems unprofessional that a developer might forget why a code was written you have to recall that developers write a lot of code, it's not possible to remember everything. The average consumer puts so much content online that it'll be fairly difficult for even we as people to remember every post we've ever written. I personally struggle with this as I have to ensure I'm not repeating myself.
This one of the many reasons why a developer should leave insightful comments in their code as well as ensure proper documentation is done on code. It can save one a lot of embarrassing standups.