That sliding weight is to balance the scale when there's a tare weight on it. For example: you want to know the weight of all your servos, RX, battery pack, switch harnes, and servo extensions. You use a small box to hold all that stuff, but you don't want to weigh the box itself. Move all the sliders to the left and zero the scale. Place the empty box in the scale. Slide the large weight to the right until the scale rebalances itself. Now you can weigh all those little parts without weighing the box.

Dr.1