They are intended to replace your hand with cheap yet effective cards who work well together to 1. fit a theme 2. to help you out when you burned through your resources, as happens often, even more when your deck built as such, or your hand is otherwise filled with Searing fire. Thus, the cheaper the cards are the more the card fullfills the intended purpose. Bloody moon replaced Moon castle in recent time due to a lack of compatibility with the rest.