What your list is worth
Email is one of the cheapest channels to own, because you are not renting access to your customers each time. Measuring ROI tells you whether your campaigns earn their keep, and revenue per email tells you what each subscriber is worth.
Those two numbers guide real decisions: how much to invest in growing the list, how often to send, and which offers to repeat. A list you measure is an asset; one you do not is just a habit.