Vouchers and coupons are really separate concepts. A coupon is used to trigger the applicability of a promotion (I.e. Enter FreeGift as a requirement to get the free gift with purchase promo). Vouchers on the other hand, are more like pre-assigned values that have residual value if not fully used. Promotions don’t fit use cases in which, the possibility of having a remaining value that can be used another time, is a requirement.