Outside of sacrificing it or destroying the aura, which not every color can meaningfully do, youve now disabled their most threatening creature, potentially for the remainder of the game
To put it another way, just because your deck beat down with creatures that one time, it doesnt mean that it is a beatdown deck
It lets you risk milling yourself out, but can give you a whole host of board presence in the late game
Any unspent or floating mana vanishes when the current turn ends, or even when moving between steps and phases of the current turn (see below)
(The same is true for card types that don't appear in most games, like plane or scheme.) You can't choose a subtype, such as creature types like Spider or Hero, artifact types like Treasure or Food, or enchantment types like Aura
Give Black Mage Lifelink with White Mages ability, however, and it becomes truly monstrous