Apple's been anti-multi document interface (MDI) forever. In MS Word, eg, on Windows, you'll have multiple documents open in windows that have a parent container window. Well, at least you did once upon a time, iirc. Apple doesn't like that, and the only place you've seen anything resembling them sanctioning its use was in their Java VM, and then only to make sure apps made with MDI (like in SQuirreL SQL, eg) would work without being rewritten.

Expose, the fancy deal where you can click a button or key and see all the windows open on your machine, gives Macintosh MDI for the first time. There's an option to see all open windows belonging to the current app with Expose, which is, for all practical purposes, what MDI was trying to give you. You'll have every window sitting in a "parent" of sorts. Voila.

There you have it, Macintosh copies Windows a few decades late. Take that, Mac zealots.