Any service accessible via SSL (https) has to have a SSL certificate, including any web server with encrypted or "secure" content. An SSL (Secure Socket Layer) certificate is a signed electronic guarantee that a particular server is the server it is claiming to be. They are used primarily (but not exclusively) as part of providing web pages via an encrypted connection. A certificate is signed by a Certificate Authority (CA) which ensures the integrity of the certificate.
A few Certificate Authorities are, by default, trusted by SSL clients (including web browsers), ie. Verisign, Thawte and GlobalSign, so certificates signed by these companies are validated without user confirmation. Until recently C&CZ signed its own certificates but now all certificates (through Surfdiensten) of servers and webapplications are signed by GlobalSign. That means that the former root certificate of C&CZ (certificate.science.ru.nl) is no longer needed.