Spring washers are a subtype of washers that are conical in shape to prevent them from resting flush against a surface without the presence of a load. They are designed to provide a spring force and absorb shock by providing an axial load that counters vibrations.
They are still used on many applications in the belief that they will will "lock" the nut/bolt to the joint and prevent loosening. The body of evidence, based upon both experience and experimental results, is that they do not prevent loosening and can be shown to actually speed up the rate of loosening in many cases.