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Tapping a creature and locking it down through its controller's next untap is a single-target Frost Breath, and on its own it would be a thin tempo trick: a sorcery that buys you one swing or one blocker for two mana. The Clue stapled to the back is what makes the rate hold up. Investigate converts the spell from a one-shot stall into a card that eventually replaces itself, so a temporary tempo swing comes with a delayed draw attached. The split between the two halves is doing real work. The tap-down is sorcery-speed and proactive, so it wants to be cast on your own turn ahead of an attack rather than held up as a reactive answer, and the Clue cushions the awkwardness of spending a whole card to neutralize a body for a single turn. The real economy is straightforward: you are not paying two mana for a tap, you are paying two mana now plus a future two-mana sacrifice for a tap and a card. This is the design pattern blue leaned on once Investigate gave the color a clean, on-flavor way to bolt card replacement onto an otherwise replaceable spell. It is workmanlike tempo-to-value plumbing, built so that even when the tap-down whiffs and the creature was never going to attack anyway, the Clue keeps the spell from rotting in hand as a dead draw.