An unused parameter can cause confusion about how a method is used and should be avoided. Otherwise, the consumer of your method may think that the value being passed has a significant impact on the outcome of the operation.
Unused parameters can often happen when the code designer simply forgot to incorporate the parameter into the routine. If the parameter was previously valid for the routine but no longer valid, you should take the time to remove the parameter from the declaration and update all calls to the method as necessary. In the event that the parameter is a place holder for future functionality yet to be developed, you should suppress this rule and describe the reason why the parameter is defined.