Even though Magento 2 Admin clearly states that Built-In Application cache is not recommended for production use, most Magento stores I found through BuiltWith don't have Varnish configured yet. Additionally, I have the feeling that most Magento 2 developers do not have Varnish configured locally while developing. While Magento 2 page cache has been developed in a way that you should be able to transparently switch caching backends, this probably will not be the case in real life therefore getting to know idea of reverse proxy could come in handy.