It's not really generally useful. You don't need to worry about it. It's sometimes used for some special applications like when you are using a Group X inside another content view.
Hello Kenneth, I have seen it in action with the free product "Author" field. http://www.seblod.com/products/10868
Took some time to understand the description and the use case. For example, you might create more fields in your "User" content type. Put them in "Intro" part. And in your Articles type you could add the "Author" field. The field will automatically pull in the "User's" intro fields. Actually something like the "Author's Info box" in this forum.
Wouldn't it be usefull if you guys show a small help text in the intro form view, that this intro view is associated with native joomla category views. I also wasted most of my time tracing how/where it is shows.