Among all blue cards in the game, this is the one that single-handedly justifies playing blue in any format where its legal, as it creates such a massive lead that opponents rarely recover once it resolves
You can target 1 "Nemeses" monster in your GY, except "Nemeses Umbrella"
Its also worth noting that Kaalia only needs to attack to get the effect (not deal damage) so its not hard to trigger, plus the creature she drops into play also come in attacking, which can easily end the game on the spot
If a particular card is being sought, the buyer might have to go through dozens upon dozens of booster packs just to come across that one card
With 140 HP, and only being a Basic Pokemon, meaning the opposition only gets one point for a KO, it's a great card to have, and it's understandable how it has a 55.06 percent win rate