The cell has a flat contact that is a thin film with an appreciable resistance, thus a flat connection will collect from a large area and have a lower overall resistance. As well a flat contact results in a thinner connection and allows the cell to be pressed firmly against the front glass panel, minimising the losses due to reflection, along with allowing it to dissipate heat out through the glass. You will find the flat ribbons are attached to the rear of the panel so as to then go to round cable, as this is a good use as it has a large current carrying capacity for the smallest area and thus the lowest cost for insulation. A flat cable is expensive in large lengths.