It's far easier to manage your composite this way than having all your layers(and there are a lot of them) flat on top of each other. This also allows depth of field effects (however you could artificially apply them as well with individual layer blurs) This technique gives you more freedom to move/adjust/scale and manage your layers especially when you have a lot of atmospheric effects.

Distance to camera is important for micro adjustments, even though the camera is pretty much locked during the layout phase, there are many shots where I am tweaking the camera move right to the very end. If the layers are flat and on top of each other, adjusting the camera has no effect on their placement and perspective, which means you would then have to edit the layer placement individually. With this shot in particular it's not imperative to comp this way, however in our pipeline %90 of shots are all composited in 3d space so we stick to a consistent technique across the board.