With interface we have a method to set the dependency.

Why do we need to put the setter method in different interface instead of client class? What's the benefit of having interface?

With interface injection (wikpedia) we have a method to set the dependency on the client as part of an interfase.

public interface ServiceSetter { public void setService(Service service); } 

Why do we need separate interface for the setter method instead of just having it just in the client class?