"Client Authentication" certificate for iPhone for school Wi-Fi
This certificate only identifies your device as a trusted known device and to be allowed on the network.

Credentials (usernames and passwords) can be stolen.  Hardware identifiers (MAC addresses) can be spoofed.  This unique certificate gets generated at the time of request is and cannot be spoofed.  So, even if someone steals your username and password and tries to access the network using their MacBook, it won't allow them onto the network.

It's a cert for authentication, not encryption/decryption.