Azorius Standstill plays a heavy suite of Counterspell, Mana Leak, and Prohibit to protect its control shell, making it hard to resolve your key combo spells cleanly. Xantid Swarm's attack trigger shuts off their ability to cast any spell that turn, giving you a clean window to fire off Survival of the Fittest activations or Hermit Druid without fear of a counter. Elvish Spirit Guide and Treva's Ruins are the least necessary mana pieces in a grindier matchup where you have time to find colored sources more naturally.