In Debian I always used Evolution not only as email client but as personal information manager (and in Evolution it's easy to add CardDAV addressbooks and CalDAV calendars), so I didn't even realize that if you don't use Evolution (for example because your distro decided to pick Thunderbird as default email client) you can't use CardDAV and CalDAV to sync your addressbooks and calendars (with GNOME contacts and GNOME calendar).
If you don't include Evolution in PureOS you should add support for CardDAV and CalDAV providers to GNOME online accounts (upstream bug report). In the bigger GNOME community this bug is not as critical I guess because a lot of users have Google accounts. But PureOS users need alternatives to sync their calendars and addressbooks. And you can't assume that every PureOS user has a nextcloud account.