Like most Pokmon cards, the value of Magcargo depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Great Encounters)
Curse of Dragon only requires one Tribute summon to get it to the field so if you need a strong monster out as quickly as possible, this can help at least and as mentioned earlier, Dragons have a lot of support to make them even stronger so Curse of Dragon is a strong Dragon monster you can send out and power up quicker than some other more powerful beatsticks
Real, but rarely a high-dollar card on its own
Yet, one card is currently sitting at the top of the meta for being a punitive counter to most decks, and it even killed a whole deck archetype in the process
This means they arent so much unique promo cards, as much as they are just a guarantee that players will get these specific cards