WCF OperationContract - What's the point of Action and ReplyAction?

[ServiceContract(Namespace = "http://schemas.mycompany.com/", Name = "MyService")]
public interface IMyService
{
    [OperationContract(Name = "MyOperation")
    OperationResponse MyOperation(OperationRequest request);
}

In this scenario, what is the point of the Action and ReplyAction ?

Edit: I should clarify my question...

How would my wsdl differ if I don't specify these parts? Won't it just use some combination of the namespace, service name and opeartion name anyways?