W3TC is a framework. The intent is to optimize the origin (the stack the WordPress application runs on) and THEN what runs “on top” of it. A great discussion around that is available here: http://websynthesis.com/wsa/wp-content/uploads/2013/09/The-Truth-About-WordPress-Performance.pdf?utm_content=buffere8e5b&utm_source=buffer&utm_medium=twitter&utm_campaign=Buffer
So yes, Cloudflare and other similar providers like Yotta and the like may apply browser caching policies, provide multiple edge locations and even minify CSS / JS or optimize images etc, but it’s important to know that as a framework, W3TC is integrated in with WordPress internals and also gives you control from inside WP Admin with regard to policies you can set around when content is purged, what is minified and what browser caching policies you have so that what any providers do is not just best practices as they define them or what a report indicates as “best” lowest common denominators that would “work” for anyone.
Since today’s websites are JavaScript-rich, offer personalization, social layers, shopping carts and other dynamic interactivity – just adding page caching or minify to a site is not enough when there’s a cache miss for some dynamic content in your site. So in addition to the object cache and the fragment cache and W3TC’s ability to manage the policies across supported integrations with Cloudflare, CDNs and WordPress itself (e.g. purging etc) W3TC can be used to reduce execution time (time to respond) to requests for things that cannot be cached in the first place, which is critical for today’s web.