If you cast a spell this way, pay life equal to its converted mana cost rather than pay its mana cost
The same is true for flying, deathtouch, double strike, haste, hexproof, indestructible, lifelink, menace, reach, skulk, trample, and vigilance. In essence, your aim is to play Creatures with as many combat abilities as possible
Lifegain also plays a major role in many white decks
Artifact Creatures, Enchantment Creatures, and older cards of type Summon are all creature spells
He also mills your opponents for each damage dealt to them by your Horrors in combat, and gives you something for free each turn from the stuff you milled