Washers are usually metal or plastic. High-quality bolted joints require hardened steel washers to prevent the loss of pre-load due to brinelling after the torque is applied. Washers are also important for preventing galvanic corrosion, particularly by insulating steel screws from aluminium surfaces.
Fasteners are used to mechanically join two or more objects together, either permanently or non-permanently. There are many different types of fastener, each with their own purpose.
Not all materials require the use of washers. But for wood and other relatively soft materials, washers are useful to protect against stress-related damage when the threaded fastener is driven into the material.