rubber washer is a ring made of rubber used in mechanical devices. It is used to prevent vibration from spreading from one part to another, reducing the noise levels.
A washer is a round flat piece of hardware with a hole in the centre that is often placed between screw or nut in order to increase the tension. It is manufactured by rubber. It is used in application that acquire a tight seal to protect from water humidity and other liquids to pass through. 
Rubber washer is more often durable and resistant to fluids.
Benefit of using rubber washer is that these are more flexible than metal washers
Rubber washer also has the potential to create a seal. In this case, the washer can be used to prevent fluid or air from leaking. This is due both to rubber's flexible nature and its water-resistant quality
Washers are often used in ways they should not be. For example, may use washers to be able to use a nut or bolt that is really too small for the associated project. Others use washers to tighten a bolt that is too lengthy for the project. Improper use of washers can cause injury or damage. Additionally, many do not choose the correct washer for the correct task. Only electrical washers should be used with electrical wire. High performance connectors like butterfly valves require certain types of washers as well. Each washer is designed to perform a specific function, and it is essential that the user choose the appropriate washer.