I know what an album is.
But I don't understand the approach of an dynamic album.

When and where is it wise to prefer an dynamic album over an real album?

Please see here:
http://www.zenphoto.org/news/how-to-use-the-dynamic-albums-feature/

I read this page and I still didn't get it.

Or it's my lousy English or I don't understand it from a technical or (for me) logical point of view.

It is really simple. Zenphoto is file system based so any album is a real physical album on the file system within the albums folder. Dynamic albums are saved searches (by tag whatever) and collect from other albums.

Part 1 is fine with me.

Part 2 is tricky.
If dynamic albums are search or tag-results, why does the backend offer me the option to create dynamic album if these dynamic albums are created automaticly by the frontend users?
Or did I mix things up in the BE - again?

I think you mix up things ;-) Loggedin users with the appropiate rights can create dynamic albums. Nothing is created automatically.

Btw, tag results are also search results.