For starters, red lets the deck run a full playset of Faithless Looting
Shadow Kin, Altar of Dementia, Mesmeric Orb and the rest
He would lead the army in Narfi's stead against the elves and, in exchange, he and his people would be protected from death
There are some very relevant cards which dodge IoK like Yawgmoth, Thran Physician, Goblin Charbelcher, Omnath, Locus of Creation, Primeval Titan or any of the new Elementals, but just looking at the list of most played cards in Modern shows how many other cards Inquisition of Kozilek hits
Its not just a useful death trigger: Brenard doesnt make an identical copy of the card or bring it back with a finality counter or something like that