A plastic example:
Most applications will have a graphic user interface where they manage persistent data (like articles, categories, etc.). If we see a form as an atomic element, there could be a versioned basic form with required fields, some behavioral information like which action to call or additional actions, and more. Not only that it could be extended by other forms, but also be forked to fit in another context. We were not forced to write the same code (create meta data, create form, create the markup, create logic like validations or other actions) all over again.
Imagine their would be a class "MyAppItemForm" which has all informations it needs for doing its job. It's extactly what some frameworks are doing with annotations. They write annotations and generate a "real class" which will be cached. But as i mentioned: Annotations doesn't seem right to me.
I don't know how else i could describe it. And of course i don't have a solution, that's the reason i thought it might be something where we can share our minds.