Where does node.js fit in a stack or enhance it

I am interested in learning more about node.js and utilizing it in a new project. The problem I am having is envisioning where I could enhance my web stack with it and what role it would play. All I have really done with it is followed a tutorial or two where you make something like a todo app in all JS. That is all fine and dandy but where do I leverage this is in a more complex web architecture.

so here is an example of how I plan on setting up my application

web server for serving views:

• Python (flask/werkzeug)
• Jinja
• nginx
• html/css/js

API sever:

• Python (flask/werkzeug)
• SQLAlchemy (ORM)
• nginx
• supervisor + gunicorn

DB Server

• Postgres

So is there any part of this stack that could be replaced or enhanced by introducing nodeJS I would assume it would be best used on the API server but not exactly sure how.