Not only do you get to counter your opponent's threat, but you also deal damage to their face
She also has a sorcery speed ability that allows her to waterbend X to give creatures you control a base power and toughness of X/X
The Wurm is susceptible to exile-based removal though, so watch out for that.
Eldrazi Incursion and Eldrazi Incursion Collector's Edition 0004_MTGMH3_CmdFace: Ulalek, Fused Atrocity 0005_MTGMH3_CmFeatur: Azlask, the Swelling Scourge 0147_MTGMH3_Vanity: Ulalek, Fused Atrocity 0143_MTGMH3_ComPorEc: Ulalek, Fused Atrocity 0136_MTGMH3_ComPorEc: Azlask, the Swelling Scourge 0151_MTGMH3_EclVanty: Ulalek, Fused Atrocity Check out the cards from Eldrazi Incursion and Eldrazi Incursion Collector's Edition in the Modern Horizons 3 Card Image Gallery
card.shuffler.shaft.rubber-band.3mf Alternate version of the shafts that can accomodate a rubber band on the disks
As for how she fits in the deck, well, theres nothing juicer than having Narset on the field and then casting Wheel of Fate