Nexus 2 allowed to re-name repositories, but did not allow changing the repository ID. Nexus 2.x repository name was to aid end users in finding the repository in the repositories list only by using an alternate description of the repository. Nexus 2.x repository ID was used in the URL for uniquely identifying what repository to perform an action against.
Nexus 3 only has a repository ID, and no name. On upgrade from 2.x to 3.x, there is no migration of repository name, only repository id.
Changing a repository ID can be useful in order to retire repositories and replace the original with empty ones.