My personal estimate is that around 20% of all HTTP traffic pass through a Varnish instance somewhere, many of which are not public-facing.
As to why people use Vanish, I think the main reason is that VCL gives you full and instant control over your HTTP traffic, and people really like that, because it lets unify heterogenous sites and pamper legacy systems as necessary.
https://trends.builtwith.com/Web-Server/Varnish
My personal estimate is that around 20% of all HTTP traffic pass through a Varnish instance somewhere, many of which are not public-facing.
As to why people use Vanish, I think the main reason is that VCL gives you full and instant control over your HTTP traffic, and people really like that, because it lets unify heterogenous sites and pamper legacy systems as necessary.