Linux

It's rarely a good idea to log in as root, doubly so if it's a system with sensitive data or services that could easily be disrupted accidentally. And even more important if multiple users log in. How will you know who broke things to teach them if they don't log in first. The only time I log in to any system as root other than a test system is when I need to sftp to access files or some other system that doesn't have a way to elevate permissions.

It's a bad practice to log in as root even for administrative tasks. You need to run numerous commands, some of hem can be potentially dangerous while not requiring root privileges. So normally you have an admin user in the sudo/wheel group and need to login to this account. Also, this adds some protection in case your key has leaked.

Is there any point of logging in with a different account?

When you edit & save a file as root, root takes ownership of that file. I personally don’t like having to run chmod or chown every time I make minor changes to something.

A door with the best lock possible is still not as secure as no door at all

It's just another way of minimizing your attack surface. It's pretty much the same as hiding behind a barrier when being shot at, you stick yourself out as little as possible.

In the same way it also helps to change your SSH port to somewhere in the high numbers like 38265. This is anecdotal of course, but the amount of attacks on SSH went down by literally 99% by just changing the port like that

Then you accept only keys, you lock down root (so the username must be guessed as well) and yeah, you're safe.

That server's root access is now vulnerable to a compromise of the systems that have the private key.

Zero-day exploits are security holes that exist and are used by bad actors, but aren't yet known to you, or anyone capable of closing the hole. The clock to patch the hole doesn't start running until the exploit is known: it stands at zero days until the good guys know it exists.

What zero-day exploits exist for ssh?

By definition, you don't know. So, you block root login, and hope the bad actor doesn't also know a zero-day for sudo.

Well, with root enabled, the SSH server at least need to verify the key, no? It's wasting CPU power albeit tiny amount.