The finish washer has an increased bearing surface and the sunken design gives a clean appearance. It's used in applications where the decorative appearance is important.

Gutter/Stove Pipe Screws - Head Type Hex Washer Head Slotted - Finish Zinc Plated with Brown Painted Head

Flat head types have flat surfaces that taper toward the screw body to allow for countersinking.

Self Drilling Sheet Metal Screw