I think it was required to support OneDrive Business or something.
We unfortunately won't change the requested permissions unless something stops working, because it's too difficult to get anything working at all with OneDrive. So if it works, we don't touch it.
Want to add, that this is what desktop Joplin says regarding OneDrive:
The application will create a directory in "Apps/Joplin" and will only read and write files in this directory. It will have no access to any files outside this directory nor to any other personal data.