It's for contacts, calendar, tasks, and notes. It's more of a CalDAV/CardDAV replacement with end-to-end encryption than a Nextcloud replacement, which AFAIK doesn't have E2EE for syncing these bits.

Also, as it doesn't directly use CalDAV/CardDAV it depends on a client app. At least on Android, the app syncs to the system calendar/contacts so it works just as seamlessly as Nextcloud's recommendation of DAVx5 or any other contacts/calendar/tasks service in that you can access the data from any standard calendar or contacts application.