The first category to look at is the most obvious one: spend X mana to cast a card, and when it enters the battlefield, you get Y energy
Some cards are literally priceless because they bring so much value to an individual that they would never imagine selling or parting with their card in any way
Collectors often prefer to buy sealed cases or join case breaks to maximize odds of pulling one, since the chance from a single loose box is very slim
Although in Ninth Edition, it was not simply printed, but replaced by its non-flying counterpart, Utopia Tree from Invasion
This example is a clear case of it: being able to continually weaken or outright remove creatures for the rest of the game, guaranteed, is too oppressive
Since Atog was popular back in the day, it only made sense that a legendary Atog would come