On your laptop you can build and test the new version of the app that needs python 1.2.4. Once you decide that's ready to go, you can push the new container onto the same CoreOS machine, so it's running both containers. Without the containers, running two versions of python on the same box isn't possible. If you had a chef script that updated to 1.2.4, you'd possibly break every other app on the box.
Containers also let you do some cool things like sign and verify a container before it's launched on the box. It should be bit for bit the same on your laptop as it is on the remote machine. Containers also boot within seconds, much faster than a VM. There have been a few tech demos running around that actually spin up a new container with a web server to service every web request, just to show how fast you can boot them. 300ms is pretty long for a web request, but it's the idea that counts.