Washers are commonly used in conjunction with a bolt, nut or screw to reduce the under-head bearing pressure when fastening mating components.
In manufacturing where assembly time accounts for a massive proportion of the overall product cost, any reduction of component assembly time can lead to large cost savings. If the washers and screws are combined, or pre-assembled together when the fasteners are first manufactured, some of the assembly cost can be transferred to the fastener supplier and taken out of the overall cost of production.