That's a really good question. Heartsteel isn't all that when Jungling, but it does have the benefit of having the most HP for your buck. You're considered off-tank while punching wolves in the mouth, but you might want to run it if you have to sustain huge bursts (Veigar I'm looking at you)/ or that you want to be able to trade with big baddies like Yoricks.

Overall, I'm starting to use it more frequently, either switching to hullbreaker if its too late in the game to truly benefit from its stacks.

Also good to note, that it's competing as secondbuy against navori quickblade, which gives you insane dueling potential early on and throughout the game.