I can tell you one place I've found them very valuable: development systems. The longer I've been in the field, the more I've learned that polluting my dev box with globally installed tools makes life painful down the road. Docker provides a nice way (especially now that they have docker exec) to spin up environments, or set's of environments with tools/stacks and not touch the host. It's lighter than a bunch of vm instances and easier to orchestrate on a small scale than most tools I've found. Docker for Mac makes that even nicer!