Prior to MailPlus there was no email client either. But now Synology have a revenue stream for people that use MailPlus and need more than 5 accounts (or 10 with a HA setup).
Before this, and still with CardDAV and CalDAV, the services were open source ports of server technology and the user had to bring their own client. There are other server services that Synology provides that there are no Synology clients, e.g.: SFTP, WebDAV, SSH, OpenVPN, L2TP/IPsec VPN, and PPTP VPN.
Most platforms include CardDAV and CalDAV support with the major exception of Android. The Google provided platform expects its users to use Gmail and other Google services. But the good news is the platform is much more open than the Apple-ecosystem so there's plenty of app stores where you can find whatever third party app you need. I'd caution to select an app store where you trust them to keep your payment credentials safe and also curate the content so that there's less chance of picking up a dodgy app build.
There is a new Synology Contacts package in beta, but this is still based on CardDAV. The main point seems to be a web GUI linked into the MailPlus and other Synology office ecosystem rather than mobile apps (at any rate that's where it is now, who knows there could be a mobile app later).