What goes in an index file?

I was wondering why the docs suggest index files for each bundle.So why do you need an index file in each bundle and what goes in the index file ? It looks like the actual files you use as post, for instance, are the other files in the bundle.

The content of the index file is what you’d see when calling {{ .Content }} in your template. The other markdown files in the bundle would need to be referenced as page resources.

The best way to understand this behavior is to make a small site and play around with it.

Regular non-bundle pagecontent/foo.md

Same page as a Leaf bundle pagecontent/foo/index.md

Same page as a Branch bundle pagecontent/foo/_index.md

All 3 pages above are identical in content. But just renaming in that special way makes them a bundle or not.