Action class can act as a model and also helper class for controller servlet called ActionServlet in struts
MVC rule Controller(ActionServlet) should interact with model
1.Action class as a Model
In small scale applications,It is good to write Business Logic in Action class.ActionServlet interacts with Action class.
2.Action Class as a helper Class for Contoller Servlet(ActionServlet)(part of controller)
In Large scale applications where Spring or EJB componets are acting as a model,There Cortoller Servelet(ActionServlet) can not interact with Spring or EJB components because its a Built_in class ,so we write the logic to interact with Spring or EJB in Action class.That means ActionSevlet interaction with the Model with the help of Action class.