it's about flatness
I used to fly. Piper Cubs, which are slow and need lots of lift, have fat wings with big curves on top. F-16s, which use lots of speed to generate lift, have virtually flat, thin wings.A fat curvy sail generates lots of lift. When the breeze is light, you want to make the sail fat by loosening the outhaul, the vang, and keeping the traveler in the middle, twisting the sail. When there's lots of breeze, you want to flatten the sail by tightening the outhaul, tensioning the vang, and letting the traveler slide to leeward.By the way, the traveler really only has effect when close hauled. When you're off the wind, there's not much difference between centered and run out.Does that make sense?