For me the main reason is consistency. I can write apps in Python, Ruby, node, etc. and configure them all the same way. The envdir program (from daemontools)

For me the main reason is consistency. I can write apps in Python, Ruby, node, etc. and configure them all the same way. The envdir program (from daemontools) is useful for setting environment variables in a shell-agnostic way, e.g., run your app with: envdir ./env python app.py See also http://12factor.net/config http://12factor.net/config