The core idea is that a creature with Prepared carries a spell with it, and under the right conditions, that creature can cast that spell
Fighting is weak to either Grass or Psychic
Nostalgia likely drives the high value of vintage Pokmon cards, as they represent a cherished part of childhood
Simultaneously, our two rival trainers metamorphose into soft-shaded 3D avatars that give them a superhero-like feel, and to quote the legendary Rayquaza, prepared take charge of [their] destiny, and knockout their opponent as swiftly as possible
It was a fragile engine back then, and had no real way to interact with the opponent (at least, before the printing of Jace, the Mind Sculptor)
If you're interested in the how rarity can impact the value or impression of a card, we've got a few guides to get you up to speed, such as Magic: The Gathering Cards, Digimon Cards, and Pokemon Cards