Themes allows you to use dreamweaver template-like functionality without needing to actually apply the template to the page.
This way you can quickly provide different themes for different sites, or even expose setting skins for your administrators or users that can be customized.
Plugins allows you to create portable functionality. This way you can design one contact form and use it on different pages or sites and organize includes into different directories and not worry about pathing issues in Dreamweaver.
It allows you to create modular functionality that you can then use in more than one place.
We'll be updating powerstore to use these concepts in the next version to show some more functional examples of how this can be used.
Let us know if you have a hard time understanding it, or what you do and don't find useful.
