Occasionally some decks will require more, while some formats (or deck archetypes) feature enough mana acceleration and cheap spells that they require less
Not only did it naturally double your mana, but it also gave you a guaranteed use for it
For example, the below Pikachu card was printed with red cheeks instead of the original yellow
It will also give you an idea of how much the game changes with new releases
The full texts of these abilities are "At the beginning of the monarch's end step, that player draws a card" and "Whenever a creature deals combat damage to the monarch, its controller becomes the monarch." (2020-11-10) If the triggered ability that causes the monarch to draw a card goes on the stack and a different player becomes the monarch before that ability resolves, the first player will still draw the card
Kaladesh (2016) had some noteworthy variations on previous themes, but also a completely new one