It may seem a bit of an odd question ... but it does have a bit of an easy answer.
I've been using W3 Total Cache on a multi-server setup for WordPress for 6+ months. For this project we decided to use
memcached and not
About WC Total Cache and CDN
In W3 Total Cache you only sync static files like the theme files, CSS and images to the CDN.
What does Varnish do for WordPress?
Dynamic content such as requests for posts and pages is managed by WordPress and your WordPress database.
Varnish will be used to cache your most commonly served pages. Rather than requesting a post / page and its content from your WordPress database, Varnish stores these pages.
This will reduce the number of database queries and give your website a bit of a performance boost.