Onur Cobanli explains:
It is obvious that the better the design the more it will sell, but it may not be always obvious to you that it is also more profitable given the same marketing budget; because cost of marketing budget per product will be less when you have a great design; so not only it sells better, it also helps you get better profit via less marketing costs; and then there is another aspect; great design can sell at a higher cost meaning even more profits; and if you sell at the same price with good design it will sell more units and therefore will also decrease per item cost based on your fixed costs not just the variable cost of marketing.