this is much more than "just a buy button"
While it's true that players primarily buy fuel because they need it, not because they desire it, the use of discounts still plays a significant role in influencing purchasing behavior. Just like in traditional markets, discounts in the game create a sense of urgency and value, encouraging players to make purchases sooner or in larger quantities. Additionally, the comparison to traditional merchandise isn't entirely accurate; even though the discount comes from the highest possible selling price, it still offers players a perceived benefit, making them more likely to engage with the game and spend. You also are going to have a large playerbase of kids with parents, where they don't spend x amount of fuel to catch a specific illuvial they need for their arena team, but rather they get 10 bucks each weekend and can do whatever they wan't with it. With a discount system they feel they get more as they should and are likely very happy about it.
Discount is also not just given for clothing, as mentioned above, you have it all over the place.... is it eating in restaurants, buying clothes, renting a car, paying bills with e-bill systems etc. ... literally everywhere, does not have to have the tag "discount" written on it