It's all to do with your personal choices and risk factor. When I book my regular flight with a certain operator I can pay for the privilege of reserving my seat of choice. However, if I wait until on-line check in (72 hrs in advance) I can select any available seat for free. Since I know that this particular flight is rarely more than half full I always wait until I check in to choose my seat. If, on the other hand I travel on a flight which is generally close to full I will pay to ensure I don't end up left with the choice of middle seats when I check in.