UW Control leans heavily on cheap interaction and sweepers like Supreme Verdict and Temporary Lockdown, so Spell Pierce is excellent for forcing through a Teferi, Hero of Dominaria or a sweeper on a key turn. Quantum Riddler gives you a resilient flying threat that refuels your hand after a board wipe, which matters since your low curve gets blown out by Temporary Lockdown. Flow State is too card-disadvantage-prone when they're holding up counters like Dovin's Veto and No More Lies, and Monstrous Rage is weak once they start wiping the board, so both come out. Play around open mana for their counterspells and hold Spell Pierce for their proactive planeswalkers and sweepers rather than their cheap removal.