The multiple passes are like smoothing a rough surface with sand paper.
First you go over it quicky with course paper to get those really high bumps (stuff you quickly recognise or already know) Then you take a slightly finer one it's still course. The second stage of activation, build on what you recognise. You continue going into each level of detail adding a bit more to you knowledge.
This works faster for sanding a rough surface (tough subject activation) than using the softest grade of sand paper (analytical reading) first.
Alex
