What is the point of uWSGI?

I'm looking at the [WSGI specification][1] and I'm trying to figure out how servers like [uWSGI][2] fit into the picture. I understand the point of the WSGI spec is to separate web servers like nginx from web applications like something you'd write using [Flask][3]. What I don't understand is what uWSGI is for. Why can't nginx directly call my Flask application? Can't flask speak WSGI directly to it? Why does uWSGI need to get in between them? There are two sides in the WSGI spec: the server and the web app. Which side is uWSGI on? [1]: https://en.wikipedia.org/wiki/Web_Server_Gateway_Interface [2]: https://uwsgi-docs.readthedocs.io/en/latest [3]: http://flask.pocoo.org