Mostly it is about the Initiative mechanic
Oracle gets away with not only being a 4 drop ramp effect but also a weak Exploration because she is self contained and a source of ongoing card advantage
Sure, most of the time, you'll roughly break even in terms of value, but by buying every drop, you guarantee that you'll end up with the drops that are worth significantly more than their cost as well
The former locks my opponents out of casting spells during combat and if a creature I control does damage to a player I get to goad their army, forcing them to attack another opponent if able until my next turn
Purphoros starts off as an enchantment before becoming a God creature once your devotion to red equals five
Theyre also great in the 99 of any artifact deck that likes to commit to the board, as they give you incremental advantage each turn