Okay, I think I get this now.
Why can’t nginx directly call my Flask application?
Because nginx doesn’t support the WSGI spec. Technically nginx could implement the WSGI spec if they wanted, they just haven’t.
That being the case, we need a web server that does implement the spec, which is what the uWSGI server is for.
Note that uWSGI is a full fledged http server that can and does work well on its own. I’ve used it in this capacity several times and it works great. If you need super high throughput for static content, then you have the option of sticking nginx in front of your uWSGI server. When you do, they will communicate over a low level protocol known as uwsgi.
“What the what?! Another thing called uwsgi?!” you ask. Yeah, it’s confusing. When you reference uWSGI you are talking about an http server. When you talk about uwsgi (all lowercase) you are talking about a binary protocol that the uWSGI server uses to talk to other servers like nginx. They picked a bad name on this one.
For anyone who is interested, I wrote a blog article about it with more specifics, a bit of history, and some examples.