[jlokier]

The transforms are specified so you can position things perfectly these days, when aligned with screen pixels.

Think of the compositing of layers of translucent windows used in modern 2d window managers, while dragging them around. Or even scrolling in a browser. Those rely on the GPU for fast compositing.

Even for 3d, think of the screen-space techniques used in games, where it's necessary to draw a scene in layers combined with each other in various interesting logical ways (for shadows, lighting, surface texture, etc), with the pixels of each layer matching up in a reliable way.