Magic doesn't always work, or at least, it shouldn't. When a fighter swings a sword, he has a chance of missing, even of really screwing up the attack (if one is using critical fumbles in the game). Why shouldn't the same apply when a cleric or a wizard is casting a spell? And we're not just talking about someone hitting a spellcaster and disrupting their spell in melee. Consider this: why would harnessing mystical and eldritch forces be more reliable than shooting a bow and arrow at someone? If you don't need Casting Rolls to make magic work, then it is!
Not only does having a constant chance of spell failure add to the verisimilitude of magic in the game, but it is also what makes gaming more fun. Always being able to cast one healing spell after another without fail can really take the *magic* out of casting spells. And the same applies to other spells that might seem more like magical rituals: what would happen if the ritual went wrong? Now that's interesting.
Unlike other casting systems though, Divinities and Cults doesn't stray too far from classic spells. Casters still cast the spells we've known about for the last 35 years, except with the twist that they may not work when you cast them. And Divinities and Cults also has options for using Boons and Elaboration, but that will remain a topic for another article.