Other than being the most rare cards in the Duel Links game, people desire them because oftentimes your most important cards in your deck are going to be UR cards
SAR vs AR vs SR Quick Comparison The clean mental model: AR is an entry illustrated card at the artwork tier, SR is a premium version of art that already exists elsewhere, and SAR is brand-new art with no counterpart, at the top of the price curve
A creature that's a copy of either has a mana value of 0
Worth a heads up before playing
If you're not using it for Double Thread, then at the very least, it's another dual type Pokemon that adds 60 more damage to Rainbow Force, and its pre-evolution, Joltik, has free retreat, so that's a nice bonus in and of itself