Forum Discussion
Why delete permission is required to rename a document in SP Document Library?
Yeah, with a standard Contribute level sans Delete, the permissions mask isn't going to match what SharePoint is requesting to rename an item (0x28 or DeleteListItems + OpenItems). This is identical to what would occur on a file system over CIFS or local (NTFS); the operation is not a 'rename', but rather a move operation. A move, of course, requires the ability to delete the source file once the move has been confirmed.
And as noted in the article, SharePoint acts similar to a file system, hence acting like an Windows Explorer move operation.
Not really an answer to your "Why?" question, but this is by design
https://support.microsoft.com/en-us/help/2505742/contributor-users-are-not-able-to-rename-items-in-sharepoint-if-the-delete-permission-level-is-removed-from-the-contributors-group
