The app is a competitor to google drive (app). It is used to upload/download, backup, syncronize (one or two way) files, media and documents between the device and the cloud. Doesn't that cover more than one of the mentioned uses? Why would FilesyncPro (example) get to have the permission but not nextcloud client? Even for media files specifically there are a lot of gotchas without full file access, like risk of location being stripped from all images synced trough the app (unless user gives media location permission) or similarly missing exif.. To upload on change it needs to be allowed to watch the filesystem

Meanwhile google drive gets to be installed as a system app