How many times have you been writing an email and had to reference something in another message? Saving a draft, opening the old email, and then reopening your draft wastes valuable minutes. The new compose pops up in a window, just like chats (only larger).
This argument is worthless. You could always hit the pop-out button and give your mail editor it’s own window. Now Gmail is trying to put a window-manager within a browser-tab.