Takedowns on enemies grant permanent Attack Speed.
Darius often struggles sticking to his targets to finish the kill. Thankfully with Phase rush a simple [E] -> Auto -> [W] will proc the move speed to allow you to position for your [Q] and chase them down.
While Darious doesn't typically suffer from mana issues the other options in the tier aren't much better. If the enemy team has multiple AP threats, take Nullifying Orb here.
Celerity pairs very nicely with Phase Rush and Darius's item build (Black Cleaver Movement Speed, Deadman's, etc.) the Movement Speed from these items grant you more AD, who doesn't love that?
Scorch's extra bit of early game damage helps you to turn your all-in into an early game advantage that you can snowball into a carry.
Triumph's bonus healing caters massively to Darius' reset nature allowing you to take over late game teamfight dunking opponent after opponent.
A huge component to Darius' ability to all-in early and take over the lane is proccing Phase Rush and running down your opponent with auto attack and building bleed stacks.