For example, you want to give a big amount for successful referral such as $20, but you have a large divergence of products in the shop in terms of price: some products cost $10 and other products cost $200. If the referred friend only spends $10 in your shop, you will be giving the referrer a $20 coupon as a reward and you will have lost $10. When you set the minimum purchase you ensure that this cannot happen. With a minimum purchase of $100, you can make sure that the referrer is only rewarded if the referred friend spends at least $100. So you ensure that the minimum profit that you will get out of your referral is $80.