Could someone please explain to me what uwsgi does? Looking at the main page, it does ... everything! It's a wsgi server, but it looks like the default configuration is still to put it behind another server like nginx or cherokee. Also, nginx for example, has its own mod_wsgi. After reading the docs, I still don't quite understand why I'd want that extra layer between my server and my code.
Take it as an "infrastructure"/"swiss army knife" for hosting and deployment. Most of its features are still topic of research in autoscaling,management, tuning and so on... Most of the users do not need that features, but others (like the Emperor) can be useful for everyone. Regarding the additional layer, is something required, as your code/framework need a way to communicate with the webserver and being healthy. Nginx's mod_wsgi is an old non official module, very funny, but it works in a very different way embedding python itself in the nginx core.