This type of refer-a-friend program is great for getting referrals and for the acquisition of referees. As it grants a discount or a perk for those referred, it is encouraging them to purchase from you for the first time.
As the referrers will probably use the discount they got if they place an order with you, it is easy to attribute the referrals to their results. In the case of a single-sided referral program, it is much more difficult to attribute the referrals to the actual sales as the referees may come back to your site later from another link than the one they got from their friend.
It is easier to track if the referral was successful, therefore you can reward only the successful referrals (these, where the referee placed an order), not all referrals.