What is a purpose of an SSH host key?

It must be sent by any SSH client in addition to a user key in order to identify the client's host.

It provides the server's identity information to connecting SSH clients.

It is the root key by which all user SSH keys must be signed.

It authenticates any user that logs into a remote machine from the key's host.

It is used by system services like cron, syslog or a backup job to automatically connect to remote hosts.