Does Sion passive serve a purpose other than cosmetic?

Used to steal the opposing team's buff level 1, then TP top.

If in a teamfight, you suddenly go from a massive CC threat to a massive damage threat. If solo, it's usually useless unless the other person is stupid and sticks around. If vs anyone with mobility or CC, it's useless.

But you know, old Sion's ult had exactly the same problems, and did not give 10% enemy max hp as physical damage or a speedboost.

It is usually best used to make the enemy team have to spend CC and damage on an already dead tank, which means your team can/should clean up the enemy while they worry about the enraged Zombie colossus that is killing them.

Try to use it with Youmuu's and Righteous Fury, you make people poop their pants

Simply put Sion's ult has the same function as Kogma's ult, and yra's ult A final F. U. to the enemy