Site-to-site is much more versatile than multi-DC communications (though this is a great use case for this NiFi's feature). S2s can link multiple clusters (or standalone instances), can even connect a cluster to itself (for data re-distribution), as well as used for MiNiFi to NiFi communication. It's also bi-directional, meaning it can be push/pull in either direction. At the end of the day this means you will be able to communicate e.g. over a corporate HTTP proxy regardless of the inbound/outbound firewall rules, there's enough flexibility to accommodate these scenarios in s2s.
Read up more here https://nifi.apache.org/docs/nifi-docs/html/user-guide.html#site-to-site