Purpose of the CODEOWNERS file

There have been a flurry of MRs to add ownership to the CODEOWNERS file but it's not clear to me what this file does. It links to this page: https://docs.gitlab.com/ee/user/project/code_owners.html Which explains the syntax quite thoroughly but doesn't as far as I can see explain how this information is used? So before I add myself to some files, what does adding ownership actually do? Cheers, Matt