I suppose it is a way to limit the scope of a promotion.
Games obtained by coupon code can only be added to your own account. If you already own the game, you are forbidden from acquiring another copy.
With discounts, games can be purchased as a gift. You get a code that never expires and can be sent to anyone. (There is a limit of 1 gift copy per user, per day though. No buying hundreds of copies at 90% off).
This has the biggest impact on DLC. With a coupon code, I can't purchase heavily discounted DLC before purchasing the base game. With a discounted price however, I am allowed to purchase it as a gift, to be redeemed when I purchase the game at a later date (hopefully at a very nice discount).
Post edited August 07, 2021 by Mortius1