If SSH is on a non-standard port, it is still possible to brute-force access to the server. You will see fewer automated attempts but you are still vulnerable to a motivated attacker who port scans you and finds the SSH port. Such an attacker is less common than automated scans but is more of a threat. With Sshguard, you are no longer vulnerable to this type of attack at all, no matter which port you run SSH on.
If you can make hundreds of login attempts per second, and you can keep at it for days/weeks/years, you can get through some pretty big password dictionaries with lots of variants (e.g. password/p4ssword/passw0rd/p4ssw0rd/etc.).
