Designer is a hybrid app which lets you mix vector and raster effects. Also, when designing for a pixel-based medium it is often useful to be able to see and think in pixels directly, even if you are using vectors to produce them. For example, if you have to draw an icon which is 32x32 pixels, if you don't pay attention to how your lines fall on the pixel boundaries you may find what you intended as a solid black 1-pixel line becomes a grey 2-pixel line.
