Magento with Varnish
When using Magento with plain old Varnish, you can configure
app/etc/env.php to instruct Magento to automatically purge Varnish caches when Magento Caches are flushed from either Admin or Cli.
Magento Cloud with Fastly
When using Magento Cloud with Fastly, there are Fastly Purge Options in Admin Configuration for Category/Product/CMS Page that when set to "Yes" instruct Fastly to automatically purge those caches when a cache invalidating event occurs. This appears to be a feature parity of the http_cache_hosts, but it's not explicitly clear.
I'm trying to understand if http_cache_hosts is even necessary when using Fastly and what is the correct way of configuring Magento Cloud with Fastly to automatically purge Fastly caches when Magento cache is flushed from Admin or CLI