The first benefit is some bots won't bother testing passwords as the SSH error message tells them the server doesn't use password auth. The second benefit is if

The first benefit is some bots won't bother testing passwords as the SSH error message tells them the server doesn't use password auth. The second benefit is if your server is compromised it's quite easy for a rootkit to hijack SSH and steal your password when you login (and then abuse that on other servers you use it), the same is not true with a key and it is much harder for a rootkit to abuse as long as you only use the key on your local machine (there are strong protections against SSH handshake MITM attacks afaik)