User blog comment:Mineko Charat Lucky/Stop Telling Me What to Build, Thanks/@comment-4057638-20131013234410/@comment-1330314-20131014124619

Warmog's makes you tougher to kill. That is the definition of a tank item. Obviously, taking it as your only defensive item doesn't make you a tank (or even tanky), but buying armor/MR afterwards multiplies your effective health by a huge amount. Stacking health is actually not a bad idea in many cases, especially since the changes to armor penetration in Season 3 made it a lot more viable, though by some point getting resistances becomes more gold efficient.

If the enemy builds health instead of armor after you've bought a, then that's not smart of them at all, but usually it pays off to have 1000 extra health when you plan on being the team's tank/bruiser. I'd like to add that if you're rushing BotRK in the laning phase before your opponent stacks health, you have absolutely no concept of the item's use.