To name two: 1) metallic washers can spread bolt stress out over wider areas of softer substrates, and 2) nonmetallic washers can provide dielectric isolation in potentially corrosive situations, or electrical insulation.
"Wat" is a Cambodian temple; the word you needed was "what."
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I agree wid da worldly answer from the previous responder, in short, whenever two different metals come into contact electrolysis begins an if you want your "Temple in Cambodia" to last don't make it outta metal's!
steel washers allows the bolt to be tighten with out damage to the mated surfaces the washer takes the damage.
Plastic can be a seal.
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