Preordain helps blue decks improve the next draw for a single mana. Scrying two lets you keep useful cards on top, move unwanted cards to the bottom, or split the pair before drawing. That flexibility is useful when you need a land, an answer, or a particular kind of spell rather than simply another random card.
Unlike Brainstorm, Preordain does not require a later shuffle to avoid drawing cards you have already decided you do not want. If both cards are poor fits, put both on the bottom and draw the next card. If both are useful, you can choose their order and plan around the card left on top for a future draw.
Spell-based decks can earn extra value from the cast. Talrand, Sky Summoner creates a Drake, and Storm-Kiln Artist can produce a Treasure to support a longer sequence. Aetherflux Reservoir also rewards another inexpensive spell. These payoffs make Preordain useful beyond the opening turns, especially when the deck wants to keep casting while improving the quality of its remaining resources.
The spell is a sorcery and replaces itself rather than creating a lasting increase in hand size. You also complete the scry and draw before taking another action. Think about the rest of the hand when deciding what to keep: a powerful expensive card may still be a poor choice if you urgently need the land that lets the deck function. Preordain works best as inexpensive selection alongside the engines that provide your sustained card advantage.
