We’re all trying to shave ms off loading times and WordPress is pretty slow without any additional tools. And Google is not happy when a site is slow.
With the fastcgi module in PHP5 it performs much better but Nginx also has it’s own caching built in. I’ve used it before on a small server serving a small amount of WordPress sites and it negated the need for installing WordPress plugins or tools such as memcache or varnish.
The basic setup is easy enough but is highly configurable. The following link on nginx’s site shows how configurable it is http://nginx.org/en/docs/http/ngx_http_fastcgi_module.html#fastcgi_cache