Strength of the Ages

Cinderhulk Junglers.

It lets them rush a large armor item (i.e. Frozen Heart or Thornmail) immediately after finishing Cinderhulk without being too vulnerable to magic damage or having a low HP pool.

It's useful on Volibear and Sejuani for their health scaling damage too.

yes

i take it on Sejuani because it gives me a chance to sit on a decent health pool with Cinderhulk, so I can buy a non-HP item high in resists and still be well off

Don't forget about jungle

alot of pros actually use this mastery on tank or bruiser junglers for the mid-game power spike which is equal to 1000 gold + slot efficiency.

Yep. Jungle tanks I use it because they get it pretty easily.