Every window looks perfect all the time, and it doesn't matter if a program is busy.
Without composition, each program repaints itself. Which means there's an appreciable lag, and if the program is stuck you can get a blank box if a previously covered program is uncovered. This can be an annoyance if you need to read something from there.
Eg, an actual example is a program being blocked by a modal dialog stops being repainted. If the dialog asks you "Enter a password" and the what for is written on the no longer repainting parent you may have a problem.
Without composition, each program repaints itself. Which means there's an appreciable lag, and if the program is stuck you can get a blank box if a previously covered program is uncovered. This can be an annoyance if you need to read something from there.
Eg, an actual example is a program being blocked by a modal dialog stops being repainted. If the dialog asks you "Enter a password" and the what for is written on the no longer repainting parent you may have a problem.