Perimeter of Circular Sector vs. Measure of Central Angle

Use this dynamic worksheet to explore the linear relationship between the perimeter of a circular sector and the measure of the central angle as in Figure 24.11. Drag the green point along the circumference of the circle and observe how the perimeter of the circular sector changes.
The radius of the circle on the left is 3 units. What is the significance of the y-intercept of the graph on the right?