My main reasons for using docker have been for portability and a development environment that is an exact match to that of production.

It's also very useful in deployment, I have different containers for my web, primary worker and mongo replicasets. Combined with Digital Ocean it makes it very easy for me to quickly spin up new servers and set them up as needed with only a few commands. There is also comfort in the fact knowing that I can do the exact same thing on linode without any modification to the container.