GarageBand is a co-called "single window interface". Apple is using that in concept in most of their apps.
Advantage
The advantage is that you stay in one window and show/hide additional windows in so-called window panes on an as-needed basis (automatically resizing the available space inside the Main Window). This keeps the screen from cluttering and you don't have to move and resize window. It also has to advantage to use and stay in the Full Screen mode as long as possible.
Disadvantage
This concept works best on a single monitor. The problem is that you can't brake-off individual panes as standalone windows to drag to a second monitor. Logic has more of a hybrid system that allows you to work in a single window but also open some windows (not all) as a standalone windows. GarageBand doesn't allow that. However there are still some windows that are not part of the Main Window (Musical Keyboard, Plugin Windows, etc) that you can drag to your second monitor. Better than nothing.

Hope that helps
Edgar Rothermich
http://DingDingMusic.com/Manuals/
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