Mono-Blue Terror relies on resolving cheap threats and protecting them with Counterspell/Dispel, so stripping a key spell with Mesmeric Fiend before they can deploy Tolarian Terror or Cryptic Serpent is valuable. Be aware that Fiend is a creature spell, so Counterspell can still answer it. Only Dispel and Spell Pierce can't touch it. Healer of the Glade's lifegain buys time against their Delver of Secrets clock while the deck assembles its own game plan. Wall of Roots and Gatecreeper Vine are defensive creatures that can't block a flipped, flying Delver and do little against a deck with few early attackers, so trimming them for proactive disruption and a life buffer is correct. Note their sideboard Blue Elemental Blast and Hydroblast have no targets in our deck, since we play no red cards, and Annul and Steel Sabotage can only hit artifacts like Lotus Petal (or a Food token from Generous Ent). Gut Shot is the card to respect, since it can kill Mesmeric Fiend, Elves of Deep Shadow and Quirion Ranger. If Fiend dies, the exiled card returns to their hand.