What happens if i didn't use NGINX with uWSGI or Gunicorn?
If your script acts like a webserver then it is a webserver and you don't need any layer on top of it.
You have to make sure though it acts like one:

listens for connections
handles them concurrently
wakes up upon server restart, etc…

Also:

handles internal connections correctly (eg. to the database)
doesn't leak memory
doesn't die upon an exception

Having a http server in front of a script has one great benefit: the script executes and simply dies. No problem with memory handling and so on… imagine your script becomes unresponsive, ask yourself what then…
Can someone brief me on what happens if I didn't use any webserver(NGINX) in front of my Application server (uWSGI or GUNICORN)?
My requirement is exposing a simple python script as a web-service. I don't have any static content to render. In that scenario can I go without NGINX?
Brief me what are the issues I will face if I go with plain app server? Max requests per second would be some 50 to 80(This is the upper limit).
Thanks, Vijay