I think one of the original ideas was to "show where something had gone" - when you minimised a window, both on Win XP (if I remember right) and OS X, it wouldn

I think one of the original ideas was to "show where something had gone" - when you minimised a window, both on Win XP (if I remember right) and OS X, it wouldn't just "disappear" but would animate down to the taskbar / launch bar. That way you get fewer people confused about where their window went. I suppose you could just about make a case that when you have a modal element like a form to edit an item, if it takes up most or all of a small screen then having an animation to expand it makes a visual difference between "you're now on a new page" and "this form has popped up, but when you close it you'll be back on the page you were before - it hasn't gone away".