I'm testing nginx with different configurations to replace an architecture working with squid + apache. I know that I can use nginx to manage static requests and load balancing but I'm interested in one particular solution that I don't understand clearly:

I'm using 2 nginx servers (balanced) with the proxy_pass setting to pass all requests to an apache server. When one client makes a request to the site one of the nginx servers process it and send it to the apache server. Now, how this behaviour could be an improvement to my system?, it seems that all requests are passing through apache and I don't see benefit at all. What happens when 100 simultaneous connections pass through nginx? The 100 connections will be going to the apache server or is some kind of internal behaviour that allows an small impact into apache?