spawn-fcgi is used to spawn remote and local FastCGI processes.

While  it is obviously needed to spawn remote FastCGI backends (the web
server can only spawn local ones), it is  recommended  to  spawn  local
backends with spawn-fcgi, too.

Reasons why you may want to use spawn-fcgi instead of something else:

*  Privilege  separation  without  needing  a  suid-binary or running a
   server as root.
*  You can restart your web server and the FastCGI applications without
   restarting the others.
*  You can run them in different chroot()s.
*  Running  your  FastCGI applications doesn't depend on the web server
   you are running, which  allows  for  easier  testing  of  other  web
   servers.