Explanation From HCIA-Security documents:
HTTPS protects web-based management by combining encryption with server identity authentication . For the browser to trust the device's HTTPS service, the device must present a server certificate that can be validated by the browser. That is why administrators often configure the device to use a local certificate (the device's certificate) that is issued by a trusted CA or whose CA chain has been imported into the browser/OS trust store. When the certificate is trusted, the browser can verify the certificate chain, validity period, and hostname binding, which helps prevent attackers from impersonating the device during login.
If a device uses a self-signed or untrusted certificate, the browser shows warnings and users may click through them, increasing the risk of man-in-the-middle attacks and credential theft. Using a CA-trusted certificate strengthens administrator logins by ensuring the management page you connect to is genuinely the intended device and that the session is encrypted end-to-end.