Why apache?

I keep seeing posts about proxies and caching but since Nginx can do this all on it’s own, I still don’t understand why people don’t just use Nginx instead of Nginx and Apache..

http://wordpress.org/extend/plugins/nginx-proxy-cache-integrator/