Two reasons:
1. Logs filling up with login failures from drive-bys masks legitimate/focused hack attempts.
2. If there's a security vulnerability found for sshd, non-standard port choice reduces the risk of drive-by scanners.
Non-standard ports don't stop dedicated attacks, but they do reduce noise that can obfuscate a dedicated attack and can reduce your exposure to uncommitted attackers.
Non-standard ports don't stop dedicated attacks, but they do reduce noise that can obfuscate a dedicated attack and can reduce your exposure to uncommitted attackers.