Why I care about Apache's mod_wsgi so much
I made a strong claim yesterday in an aside: I said that Apache with mod_wsgi is the easiest and most seamless way of running a Python WSGI app, and thus it was a pity that it doesn't support using PyPy for this. As I have restarted it here this claim is a bit too strong, so I have to start by watering it down. Apache with mod_wsgi is definitely the easiest and most seamless way to run a Python WSGI app in a shared (web) environment, where you have out a general purpose web server that handles a variety of URLs and services. It may also be your best option if the only thing the web server is doing is running your WSGI application, but I don't have any experience with such environments.
(I focus on shared web environments because none of my WSGI apps are likely to ever be so big and so heavily used that I need to devote an entire web server to them.)
Apache is a good choice as a general purpose web server in the first place, and once you have Apache, mod_wsgi makes deploying a WSGI application pretty straightforward. Generally all you need is a couple of lines of Apache configuration, and you can even arrange to have your WSGI application run under another Unix UID if you want (speaking as a sysadmin, that's a great thing; I would like as few things as possible to run as the web server UID). There's no need to run, configure, and manage another daemon, or to coordinate configuration changes between your WSGI daemon and your web server. Do you want to reload your app's code? Touch a file and it happens, you're done. And all of this lives seamlessly alongside everything else in the web server's configuration, including other WSGI apps also being handled through mod_wsgi.
As far as I know, every other option for getting a WSGI app up and running is more complicated, sometimes fearsomely so. I would like an even simpler option, but until such a thing arrives, mod_wsgi is as close as I can get (and it works well even in unusual situations).
I care about WSGI in general because it's the broadly right way to deploy a Python web app. The easier and simpler it is to deploy a WSGI app, the less likely I am to just write my initial simple version of something as a CGI and then get sucked into very peculiar lashups.