If the card has any mandatory additional costs, you must pay those to cast the card
Minwu helps your little Soul Sisters grow large enough to jam, while Aerith can find a home in Legendary Matters decks in addition to Cleric and lifegain decks
When Purphoros enters the battlefield, your devotion to its color (including the mana symbols in the mana cost of Purphoros itself) will determine if a creature entered the battlefield or not for abilities that trigger whenever a creature enters the battlefield
With the release of Winterspell, color identities are both redefined and strengthened, bringing that distinct understanding of what each ink color represents, while also adding a bit of flair
Its just a card that will almost always need to be picked up and read for casual players
They make the decks make sense both mechanically and thematically