Docker is amazeballs for things like testing your application in multiple environments and software versions. You can write one Python library and then automate the execution of that same test suite in 10 different Linuxes, and 4 different Python versions -- all as part of your CI.

It's also handy in this age of "spin up a thing to take care of X" -- though I tend to try to avoid having too many moving parts in an application. Still, being able to stand up a caching server, queue server, and data server -- all in about 30 seconds is really handy for development.