Varnish is the ideal solution to relieve your backend systems by high-performance caching. Especially together with CMS systems written in PHP, which are infamous for performance, it's possible to create solutions with more than convincing load times.
The clou: by way of the flexible configuration language it is possible to create even more specialized solutions for cases where the backend does not have a sophisticated cache handling.
With Edge Side Includes (ESI) you can cache the uncachable—the dynamic portion is replaced live on every response. A perfect fit for example to display stock prices on a homepage so the backend can withstand even a large amount of concurrent visitors.