I may have found an answer here: https://devcentral.f5.com/questions/difference-between-root-cert-intermediate-cert-and-ssl-cert
One more thing one the public key of your server certificate. There is the same mathematical relationship as described above between the public key and private key of your server certificate.
It will be used during the SSL handshake. The client encrypts some key material servers public key (retrieved by the client out of the server certificate) and sends it to the server. It can be decrypted by the server using the private key. If the server does not have the matching private key it cannot decode the key material and the handshake brakes.