Takedowns on enemies grant permanent Attack Speed.
WhyGrasp of the Undying
Cho'Gath is known for getting insane amounts of health and Grasp exemplifies that, giving even more health and damage for being so tanky.
All that stacking max health allows you to easily do half a tower's health with a single auto-attack with Demolish
Effective health comes from multiplying health with resists. Grasp and and you ultimate stacking health pairs very nicely with Conditioning to make you very tanky.
Overgrow multiplies and scales incredibly well as you continue to gain health from Feast stacks and Grasp health gained.
As a big beefy front-liner, Cho'gath absorbs a massive amount of damage in teamfights while standing in the front line. Triumph allows you to pick up assists with your AoE and still stay heathy and unkillable.
Alacrity's bonus Attack Speed is incredibly useful in fights where you are looking to get all 3 [E] auto-attacks off.