Like most Pokmon cards, the value of Cacnea depends on a few consistent factors: its rarity, the printing and finish (regular or foil), its physical condition, and the set it was printed in (Crystal Guardians)
Essentially all the addictive gameplay elements, filling up an entire index/compendium, befriending a diverse cast of creatures, and becoming the world's best tamer/trainer, are all locked inside the Nintendo vault
So not only are you swapping out your Rokkets for better ones, but because you destroyed a monster in the process, youll get to special summon a Rokket from your deck during the end phase
But I really enjoy the stories these cards tell by the artwork
While Edgar Markov's other ability definitely makes attacking with it successfully valuable, just casting Vampire spells in general will amass a favorable board state