Izzet Delver pressures with Dragon's Rage Channeler and Murktide Regent while holding up Daze and Force of Will, so we want cheap removal that answers their clock instead of discard spells that whiff once they're past the early turns. Bitter Triumph kills Murktide Regent or Dragon's Rage Channeler outright at instant speed, dodging their counterspells' tempo advantage. Collective Brutality's -2/-2 mode also kills Dragon's Rage Channeler and its discard mode can strip Lightning Bolt before it goes to our face. Unmask is clunky against a deck that empties its hand quickly into cantrips, and Cabal Therapy is too slow against their aggressive clock, so both make room for proactive removal.