League of Legends (LoL) Question: Sion and Tytanic Hydra

Why do people consider this item to be an ''core'' item in the int Sion build you can go?

I got flamed for not building it and am wondering if there is something I am actually doing wrong or if I'm right into thinking its not that good of an item on Sion.

I play alot of Sion and if there is anything you dont need as Sion its more waveclear.
Especially if you get miniondematerializer (my secondary rune tree is generally just for more sustain and pushing on Sion) you have it even more easy so is there something I am missing here?

Does it interact with my ult/passive or anything else in a way I just havent noticed and am I missing a ton of damage (both to players and turrets) by not building it?

Cause I tried building it and it seemed super useless compared to when I just go full tank right from the getgo, especially since Sterak's Gage offers damage output aswell as tankyness.

And yes I realize that the damage output increases when I get more hp but is that the sole reason why its considered core?

Cause I feel like spending gold on items that help me surive the damage an opponent can do is way more valueable then spending it on damage since removing the opponents kill pressure means I have an free lane unless their jungler starts babysitting.