Why should I really disable root ssh login?

You answered your own question. By disabling remote access to root in *Nix or administrator access in Windows, you make it that much harder for someone to gain privileged access to your computer. If someone steals or brute-forces your non-privileged account password, then they only have limited access.

root is a dangerous account since it can literally do anything it wants on the system. You want to protect it from unauthorized access as much as possible.

By disallowing root logins via SSH, you require 2 passwords for someone to gain root, instead of 1. If someone is trying to guess or crack your passwords, this doubles their workload.

@Daniel B. in the comments is right, using keys are better than passwords, if they are passphrase-protected.

The above still applies if you only allow passphrase-protected keys instead of passwords - and still disallow root from logging in even with a key. So even if you use keys instead of passwords there is a benefit from disabling direct root login via SSH.