Varnish is a cache server technology: which need to be additionally installed, configured and maintained on your server.

The Extendware Full-Page-Cache (FPC) does not need or use Varnish. In fact, FPC is a much better solution for a Magento store than using the Varnish caching solution. 

The reason Extendware Full Page Cache (FPC) is a better solution than Varnish is that - like Varnish - FPC does not use the database for most requests, so it reduces load on the database.

However, unlike varnish, FPC has a high cache hit rate (percent of requests served by cache) that Varnish can not match, due to the design of FPC and its native use of Magentos platform.

Since Varnish is a general caching solution and not built for specifically for Magento, it is not as well integrated into the platform and that allows FPC's custom Magento-based solution to outperform Varnish in almost every scenario. Additionally, using Varnish as a solution will increase the amount of installation and maintenance for your store, is difficult to debug, and increases your TCO if you have to use Varnish Plus (they don't even post their price, which means its probably pretty high). 

In addition, Varnish has limitations with flushing. For example, the whole varnish layer cache needs to be flushed when there are any changes on your store. FPC can flush selectively and not affect the cache of your entire store for when only one category or page needs to be changed.  

As a result of these Magento specific capabilities that come out-of-the-box with Full Page Cache that are unavailable in Varnish, customers using FPB on their Magento stores generally have better overall speed and experience.