Why do they ask if you want fries with that? "Elmer Wheeler was a sales trainer nearly a century ago. He got hired by a chain of drugstores to increase sales at the soda fountain. In those days, a meal might consist of just an ice cream soda for a nickel. But for an extra penny or two, you could add a raw egg (protein!). Obviously, if more people added an egg, profits would go up. Wheeler taught the jerks (isn't that a great job title?) to ask anyone who ordered a soda, "One egg or two?" Sales of the egg add-on skyrocketed." From who else but Seth Godin.
If 10% of the people buy the add-on, your ARPU increases, revenue goes up and profits should increase (unless you sell underwater).