Takedowns on enemies grant permanent Attack Speed.
WhyGrasp of the Undying
Tahm's incredibly strong slow on [Q] allows him to chase down and trade incredibly well in lane with auto attacks. The stacking health on Grasp further empowers his passive.
More autos = more damage = more health = even more damage
Bone Plating allows you to absorb a ton of damage as a front-line tank and take aggressive early trades.
Second Wind's mid-fight healing paired with your large health pool, especially when paired with Bone Plating, makes you an incredibly durable tank in skirmishes.
Overgrowth is a rune that allows Tahm to scale, both defensively and offensively, incredibly well. The bonus stacking health further empowers your autos and abilities thanks to Tahm's Passive.
Triumph is a rune that is deceptively good in it's ability to sustain in teamfights. If you're playing frontline and soaking damage, picking up assists will massively heal you to keep moving forward.
Especially during the laning phase, Tahm relies heavily on the ability to take trades by getting as many autos off as possible. The faster autos means more damage from passive, Grasp, and faster passive stacks.