Disallowing explicit `root` login makes it harder for attackers to guess the usernames which have sudo access
Or rather, it forces attackers to enumerate other usernames because the default root account isn't available to brute force. In general, when password authentication is in use (which it should never be for SSH), then disabling the default user account acts as a simple way of mitigating brute force attacks. If best practices are followed and passwords are not used to authenticate to SSH servers, then this additional measure might be moot. Even so, I can't think of a reason not to disable root SSH login anyway, even if password authentication is disabled. A layered defense assumes the possibility that other defenses might fail. Noone wants to be the person who wrote off an additional defense measure because they didn't think it was possible to exploit, only to have a new CVE come out to prove them wrong.
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Or rather, it forces attackers to enumerate other usernames because the default root account isn't available to brute force. In general, when password authentication is in use (which it should never be for SSH), then disabling the default user account acts as a simple way of mitigating brute force attacks. If best practices are followed and passwords are not used to authenticate to SSH servers, then this additional measure might be moot. Even so, I can't think of a reason not to disable root SSH login anyway, even if password authentication is disabled. A layered defense assumes the possibility that other defenses might fail. Noone wants to be the person who wrote off an additional defense measure because they didn't think it was possible to exploit, only to have a new CVE come out to prove them wrong.