Original: Internet standards documents (RFC 1033, RFC 1912 Section 2.1) specify that "Every Internet-reachable host should have a name" and that such names are matched with a reverse pointer record.
Simplified: RFC 1033 in the "INSTRUCTIONS" section, and later, RFC 1912 Section 2.1 states "Every Internet-reachable host should have a name" and "Make sure your PTR and A records match".