We need some kind of Caching system to deliver quickly the pages, it will help to improve the site performance and usability. Drupal has a default built-in cache system but it is not enough for some of the complex sites, So we are choosing some kind of complex caching systems like Authcache, Memcached, Varnish. Authcache works with both Memcached and Varnish, we can use them as a backend of the caching process.