I think the logic is that you can search through email for things while composing. e.g. I'm composing an email and referring to something in another email I got a few days ago. I can search for that email, read it, and then get back to composing. Perviously I would have had to save the email I was composing as a draft, find my other email, then go back and open the draft.
With the "old" compose UI, you can/could click the box-with-arrow button to transition into a new browser tab or window. Solves the same problem without trying to re-invent windows in HTML. I'm guessing this didn't work (well) on Chrome OS or something?
