Modal spells let you choose one or more modes as you cast them. Charms, Commands, Confluences, and mechanics like escalate and spree all offer flexible effects that adapt to whatever the game needs.
Kolaghan's Command and Abzan Charm provide efficient, flexible interaction, while Mystic Confluence lets you pick the same mode multiple times. Riku of Many Paths rewards casting modal spells with extra effects, making a deck full of options a strategy of its own.