What's the purpose? Strange login attemps "sshd[***] Received disconnect from **.**.**.**: 11: Bye Bye [preauth]"

This error rises from a fatal error in the authentication process (see monitor.c of OpenSSH versions 6.1p1+).

It is likely that the attacker is using some custom code to brute-force the server which is ending up in malformed authentication requests being sent, resulting in the server killing the connection. So from the code it appears they are in fact trying to login, but the server doesn't like how they're attempting that.

As such, these log entries aren't anything to worry about unless you think you are likely to be a targeted victim for any reason (in which case you should be taking extra precautions such as refusing password-based logins).

In any case, I suggest you install the simple fail2ban program if you haven't already which will significantly hinder cookie-cutter brute-force authentication attempts.