For many a maker newbie, the use of flat washers in fastening might seem superfluous. Yes, you can assemble many things without using them, but washers are there for a reason. Here are their main uses:
Load distribution: Their most common use of a flat washer is increasing the size of the bearing surface that a threaded fastener would otherwise provide. The washer distributes the load around the fastener and can help prevent the stressing, splitting of softer materials, like wood.
Spacing: Washers are sometimes added to reduce the length of a fastener so that it can fit a given depth.
Absorbing vibration: Plastic, rubber, fiber, or similar materials are used for washers in assemblies prone to a lot of vibration. Washers help absorb that vibration.
Leak prevention: Washers are often used where water or other liquids could otherwise leak around the fastener. Like vibration-absorption, these washers are usually rubber or other soft, conforming material.
Cosmetics/finishing: Washers can also be used to create a desired look in the design of a finished piece.