Amazon promotions do not effectively limit the quantity that a user can buy with a discount code. This means that a buyer could buy out your entire inventory if you offer a steep discount. MAX allows you to limit the quantity that a buyer can purchase a the listing level
Other merchants can use a trick to find out your sales. By limiting your quantity per seller you eliminate that ability for sellers to determine your sales.
Learn more about why you might need MAX, and why we offer it for FREE