Benefits of Varnish vs nginx/ncache?

[James Thornton]

When you have nginx as a reverse proxy to your application servers,
what are the benenfits of adding Varnish as the caching layer between
nginx and the app servers (as depicted here
http://www.heroku.com/how/architecture) vs having nginx perform the
caching, now that ncache is built into nginx?

For example, why is (nginx -> Varnish -> uWSGI) better than (nginx
w/caching -> uWSGI)?

I have read http://www.varnish-cache.org/trac/wiki/ArchitectNotes, but
I'm still trying to understand the differences in this context.

Thank you.

- James