There's a massive amount of Apache modules and installs of stuff that has various dependencies on Apache where it's simply not worthwhile to expend any effort to convert it to Nginx, but where it's worthwhile placing in Nginx in front if you prefer Nginx for things like caching, load balancing etc.
That was the original way (at least the most common way) to do CGI. Nginx as a reverse proxy / load balancer / SSL terminator / static file server pointing to a backend running apache for mod_cgi or mod_fastcgi. And a lot of people would have both web servers running on the same server. You already have a web server, why do you need two?! Just pick one!
I'd say the most common use-case (particularly given the ubiquity of PHP on the web) is to support mod_php for people not wanting to move to php-fpm for whatever reason - often some weird legacy reason, or sometimes just familiarity and change overhead.
