If you have multiple people on your team, a common dev environment can be a huge time-saver. New developers can install an identical environment quickly.

Another benefit is that your project's development environment is self-contained. It's not going to clash with anything else on your system, and will even work across platforms.

More direct question:

Our team uses virtualenv and pip install from requirement lists. What additional benefits does Docker offer?

You can do a pip install of python bindings but not the source package itself. For e.g., pip install redis gets you python bindings for the redis but not redis itself. With this setup, you can install a specific version of redis, run it as a service with your custom configuration right out of the box.

That's a good benefit for consideration, thanks for the answer.

I'm guessing easy resource allocation, either through local machine quotas/cgroups or the easier ability to move specific services to different machines as you need to scale them.

Possibility to break things, use multiple versions at the same time, testing dangerous operations, seamless deployments, etc.

Multiple versions at once is a big sell for me. At the moment I have 3 different MySQL installs to handle different work and side projects. With vagrant, this is practically trivial to maintain.

One reason why I started to use Docker is that we work in a SOA environment, and eventually bundling our own services as Docker containers would make it much easier to replicate our production environment locally.

For one, you can mirror your production environment in dev.