Mono-Blue's game plan revolves around Stasis backed by Counterspell and Chain of Vapor, so Xantid Swarm lets you attack and shut off their ability to interact on your turn, which can be the key to sneaking through the Stasis lock before it locks you out. Replenish recurs an enchantment if Counterspell counters something important like Sterling Grove or Opalescence. Parallax Wave and Elephant Grass are largely dead since this deck has almost no creatures to interact with, so they're cut for more proactive answers. Push to resolve Enchantress's Presence engine pieces quickly since raw card advantage out-grinds their Impulse and Gush card selection.