Topdeck decks manipulate the cards on top of their library because other effects care about exactly what is lurking there. Scry, surveil, rearrangement effects, and cards that let you play from the top of your deck all provide control over future draws.
Counterbalance can turn topdeck manipulation into countermagic, while miracles reward placing the right spell atop your library before the next draw. Bolas's Citadel instead lets the top card become an extension of your hand at the cost of paying life to cast the spell.