Because Ruby 1.8 threading sucks you pay a large memory price (ie a process) for each concurrent request in flight. A fronting proxy allows your backends to write out the response as fast as possible and move on to another request while the proxy spoon feeds the response to slow clients.

Also, nginx is going to be more efficient for serving static files, though most larger apps will have broken such requests out to a separate set of domains likely serviced by a cdn.