Magento2 officially recommend to use Varnish on production as it's much faster than the built-in cache, but they don't describe why it's better than the Full Page cache and what are the missing elements,
so can anyone describe what are those things which are missing in Magento2 built inn cache and are available in Varnish? What are the core differences between these two caching mechanisms in Magento? >
the answer is more about Varnish in general, if you read from Varnish page - it was specificly built as HTTP accelerator.
varnish have direct memory storage access, varnish doesnt care about backend state.
while other caching "mechanisms" are using magento backend and database to determine the appropriate tags and keys.
for example you have some ad campaign , you can send link to static cms page, and cache it in varnish. your server will barely notice 1k requests.
Magento native cache is stored on disk under var/cache. Varnish cache is a separate process with a block of allocated RAM/memory to store its cache.
Since disk access (even SSD) is always slower than memory access, memory (Varnish) will always win!
