Some of the workstations I run are sometimes used by multiple people. Having multiple people share an account is bad for security so having a guest account for guest access is convenient.
If a system doesn’t allow logins over the Internet then a strong password is not needed for the guest account.
If such a system later allows logins over the Internet then hostile parties can try to guess the password. This happens even if you don’t use the default port for ssh.
This recently happened to a system I run. The attacker logged in as guest, changed the password, and installed a cron job to run every minute and restart their blockchain mining program if it had been stopped.
In 2007 a bug was filed against the Debian package openssh-server requesting that the AllowUsers be added to the default /etc/ssh/sshd_config file . If that bug hadn’t been marked as “wishlist” and left alone for 11 years then I would probably have set it to only allow ssh connections to the one account that I desired which always had a strong password.
I’ve been a sysadmin for about 25 years (since before ssh was invented). I have been a Debian Developer for almost 20 years, including working on security related code. The fact that I stuffed up in regard to this issue suggests that there are probably many other people making similar mistakes, and probably most of them aren’t monitoring things like system load average and temperature which can lead to the discovery of such attacks.