The Lugia holo from Neo Genesis is a great example because it is so hard to find in good condition
The name comes from Ravnicas guild of the same colors, but it can be used to talk about cards in any set and plane thats those two colors
In short, you want to balance the mana value of your spells to ensure you have effective spells to cast throughout the game
Humility/Prayer was so monstrously effective at making the game entirely about protecting the lock, having a Counterspell that could buyback was backbreaking, even if it required discarding multiple cards to do so
And if booster packs were already expensive back then, imagine how unaffordable they must be to children now, who have all the time to long for cards and not the money to buy them
Facilier, Charlatan x2 Dr