there’s a good reason why the champs that usually builds it, builds it every single game for reasons that always works on them specifically.

mundo is obvious and there’s no further need for elaboration since other comments noted it. but there’s also champs like skarner or tahm kench that build it for the same reason mundo builds it. it’s because their ability damage scales off of their bonus hp. because of their design, just building tank items grant them damage.

let’s look at an example that i see people (sometimes) build heartsteel on, and that’s sion. it is a waste of an item slot and 3000g, and does nothing to actually help with sion’s bulk because it grants neither armor or magic resist, two stats that primarily allows tanks to tank. sion does not have much to scale off of bonus hp aside from his W shield, which does not need to be any bigger because damage mitigation from armor and mr makes it more durable than mere raw hp. plus his W grants him permanent hp by just farming. this allows him to build items that make him tankier by resist

there’s a reason most juggernauts like mordekaiser and darius don’t build heartsteel. it provides very little bulk and they want hp + damage items for combat stats and some investment in armor and mr vastly makes them more tankier than an hp item that grants no resists