A topic came up in my private telegram group on why we use Docker, so I'll just pasting back some of the answers here.
AC: Why Docker ? (because docker is just an additional layer of complexity, and only helps when you're doing a lot of things repeatedly, at scale. Otherwise, it's just a disk space sink). What do you wanna do with docker?
AS: It can be "Just because" guys... I dockerized my vim so from any pc I use I can have same plugins and everything. Like 60 MB compressed image and 130 uncompressed. But that's because I compiled vim with huge flag and threw in python perl lua and ruby otherwise it's much much smaller. My base vim with big feature flag is only 30MB uncompressed.
AC: The reason why "Why Docker?" is asked is to ensure you can see the benefit in containerizing your use case. Otherwise it's just so much more painful.
https://alysivji.github.io/php-mysql-docker-containers.html is already a good start. (He used Docker 'coz he don't wanna hunt down and install PHP-7.2 just for running/testing wordpress).
K: We're python shop so no proper php env. But sometimes customer come with their php script for help, so docker is the easiest to quickly test the script.