Reasons to build it
1. Makes your resists more effective. Some math behind it is that to think of each stat of one resist as 1% more effective health against that particular damage. Say you have 100 armor then getting warmong's armor grants an extra 1600 effective health against physical damage. Typically most tanks have around 200ish armor and 110 mr after their first 2 major resist items grabbing a warmong's alone grants an extra 2400 against physical damage and 1680 against magic damage.
2. It grants the most health regain in the game and grants a way for tanks to heal fully after activating its passive Warmong's heart after 6 seconds of not taking damage. It makes the item great for skirmishes or tanking a tower then backing off to regenerate the health lost and be able to take a teamfight afterwards or being able to back out of a teamfight then come back in afterwards, along with the dozens of other situations its useful for.
3. It is one of the few tank items that has cooldown reduction on it to help cap your cooldown reduction.
4. It synergizes extremely effectively with items like sterak's gage, Titantic hyrda, and Gargoyle's stone plate and make future resist items way more effective then just buying 3 armor and health items alone.