They also provide an incentive for existing customers to share

 A referral code is a unique code that a company gives to its customers to share with friends and family. When someone uses the code to make a purchase, both the referrer and the person who used the code can receive a reward or discount. Referral codes are a popular marketing tactic for companies to increase their customer base and encourage word-of-mouth recommendations. To get more information visit #referralcode.