It was at my very first job as a developer that I learned about code reviews (and that I intuitively hated them, a characteristic I share with many developers I’ve then since learned). We wrote and maintained software that automated tax preparation (yes, you may in fact be using services that contain code I touched long ago – caveat emptor ;-)) and going over code was an important part of the process. In addition to making sure we were following coding standards with respect to source formatting there was also the opportunity for improvements and to find potential mistakes that might be disastrous given the nature of the software we developed.
Fast forward a number of years (right, like I was going to tell you how many) and code reviews are still a part of the development process. The thing is that the reasons for reviewing source code have expanded, primarily thanks to the success of open source.