sshd(8): Added support for multiple required authentication in SSH
protocol 2 via an AuthenticationMethods option. This option lists
one or more comma-separated lists of authentication method names.
Successful completion of all the methods in any list is required for
authentication to complete. This allows, for example, requiring a
user having to authenticate via public key or GSSAPI before they
are offered password authentication.

I found interesting article about Google authentication http://www.heitorlessa.com/ssh-two-factor-authentication/

Why multi factor authentication important? For example it is very easy to require  both an ssh public key and a password to login.   If you don’t have the public key, you will never see  the password prompt window.