Same question here, but I think we got a clue : you can do more things with packages. It means that importing package will allow you to init for example a fresh install f eZPublish for a particular way or add just some content in an existing site.
eZNewsletter is the perfect example, the packages are creating content classes, content and special configuration like a new siteaccess. It is a way to programmatically create content and configuration. However, I haven't found any docs on how flexible this functionnality is, and if you can by this way export things like roles and policies or workflows...
I haven't had any time to experiment with this topic ever since, but it's too weird that this is so not documented (both packages alone as well as newsletter packages). If you find anything, please share!
As far as I know, a ezpkg file is a zipped xml.
The XML is formatted to receive content class definition or content.
For extension package and site design package, I'm not sure that it works like this. I can try it and share.