There are a lot of qualitative reasons: it helps spread knowledge of changes in the code, it helps maintain a consistent style, shares knowledge, etc. However

There are a lot of qualitative reasons: it helps spread knowledge of changes in the code, it helps maintain a consistent style, shares knowledge, etc. However when it comes to quality assurance and correctness (if these terms collectively mean, preventing defects) then there's little evidence that it is an effective practice [0]. If the changes proposed are less than a couple hundred lines of difference and the reviewer is only reading one every couple of hours there's small but significant chance that they might catch an error. Humans are simply bad at this task. [0] https://sail.cs.queensu.ca/data/pdfs/EMSE_AnEmpiricalStudyOfTheImpactOfModernCodeReviewPracticesOnSoftwareQuality.pdf https://sail.cs.queensu.ca/data/pdfs/EMSE_AnEmpiricalStudyOf... Why does the tech industry still rely on CR for this purpose? Probably because running empirical studies is time consuming and expensive. Instead we rely on the intuitions, experiences, and feelings of people, advice we get from others, etc.