You can proxy requests from nginx to backend application servers, but you cannot host those applications on the nginx.
Whereas with Apache2, PHP/Python/etc. interpreters can run directly in the Apache worker processes. Similarly, you can also run Nodejs/Ruby/Python web application as part of Apache with Passenger module.
Yes - it matters a lot who is in control of the code execution. The traditional way was to run the code from or as a child process of the webserver process. FPM, WSGI, etc. move that into a completely separate and unrelated process. Crashes, hangs and security issues are a much smaller problem in the latter case.
