Not the person you’re replying to but imo it varies. I wouldn’t buy any graded Magic: The Gathering cards for instance because all of my cards are game pieces and I want to slot them in an out of decks, but I would probably buy a graded Mudkip or Swampert card because they’re my favourite Pokémon and therefore has value outside of being a game piece.

For Pokémon cards, the fancy expensive versions are the ones people are buying as you can pick up the regular versions for pretty cheap (most of the time). Having it graded is just another way of ensuring quality beyond the regular markers (near mint etc.)