Varnish is a reverse proxy
working with apache, with all traffic go thriugh Varnish cache first
if cache missed, Varnish will be the one to grab content from Apache
It is because Apache ignore file types
For example, deliver a jpg by Apache will load mod_php into RAM
the same applies to CSS and JS files
This will cause massive RAM footage
And Varnish cache help greatly helped these static files with less resources drain
and improve overall performance

My target is install Varnish with minimum downtime
Remember to backup the config files before continue