Containers are easier to deploy when you have more than a couple of servers, they provide better separation between services, are simpler to move, and you can have the same containers in testing as you will later use in production. All that without the performance penalty of virtualization.

I haven't used dokku and don't know what it specifically adds.

Dokku lets you push apps to your docker server through git, and sets up a subdomain to point to that container.

Just read a few paragraphs on Dokku and Docker to get an idea. He has included these convenient things called "hyperlinks" for those topics in the article. Those solutions aren't particular to Python, for starters.