For instance, sometimes I want to monitor the status of a running backup job while I go on to change something elsewhere in the UI. As it stands, every pop-open window (even the "about" dialogue) is 100% blocking against doing any work in the main window.
I'm sure there are some edge cases where making a change in a secondary window and the main would break things. That's fine - block those. But items like the Job Statistics window that are effectively read-only should be able to background without any trouble.