Most screws and bolts are designed to go into a material and tighten down. In doing so, the head of the screw or bolt will exert pressure on the material screwed into. With wood and soft materials, this can cause damage that becomes noticeable. By inserting a flat piece of metal, which is much harder than wood or plastic or similar, the pressure of the screw head is then distributed widely, reducing the damage caused by tightening down.
The standard washer is a flat disk of metal with a hole punched in the middle. The user should make sure that the washer chosen has a center hole as snug as possible to the screw being used. Too small and the screw won't go through; too big and the benefit of the washer being used will be lost as the screw will press through to the soft material beneath.
Screws that come with flush heads can be very useful for surfaces in which they must not protrude very much. A good example would be the back of a door or frame in which the door hing must be installed and screwed in. However, such screws still need a washer. As a result, washers that have pre-sunk middles are used, providing a flush look when the screw is fully installed through the washer.
Screws that a user doesn't want going anywhere or coming loose require one of two options. You can glue the screw in using wood glue. But if glue isn't applicable, then a snap-lock washer is the next best trick. These washers are split on one side and are pressed so that one washer side is higher than the other. As the screw tightens down on this kind of washer, the upper edge creates a tension and the screw stays tight once all the way in.
For screws that are not made of metal or use softer material, metal washers may not be compatible. Instead nylon washers are used or rubber ones. Either works well and creates the same benefit a metal washer provides. Nylon washers are usually white and look like almost opaque plastic disks. Rubber washers usually come in black rubber and are flexible and bendable.
Practice Builds Experience
If the whole idea of using washers is new to you, practice on an old piece of wood using different screws and washers. Use a battery drill with some good torque, pre-drill a hole in the wood and then drill in screws using different washer types. With a bit of practice, you will get the feel of how washers work and why different ones are used. Then you can apply this knowledge firsthand on your next home project.