TO-220 rectangular washers
The use of the rectangular washer helps to spread the pressure from the bolt head evenly across the tab. The tabs are typically copper and can deform as the bolt is tightened.

The rectangular washer would typically be steel and less susceptible to deformation as the bolt is tightened.

Without the rectangular washer any deformation can cause the tab to get decreased surface contact with the heat sink surface which increases the case to heatsink thermal impedance.