The rasterization layer is necessary to flatten vector graphics into a flat image for on-screen display. Even when images have already been rasterized, graphics systems typically composite multiple rasterizations.
This layer is an immediate mode (paint directly to a buffer) phase that grants any GUI the ability to be resolution independent and hardware accelerated. Rasterizers (2d) commonly achieve hardware acceleration by projecting flat textures in a 3d space using an orthogonal camera. This contrasts with software-accelerated systems that are using algorithms to convert shape primitives into pixels. This also has the advantage of using the camera transforms to "zoom in" and "zoom out" of a scene.