Sightseer was a draw Supporter that had a different way of doing things. You could discard as many cards as you wanted from your hand and then draw cards until you had 5 cards total. You couldn't play this if you couldn't draw any cards from this, so you were required to get your hand down enough to take something.
While Sightseer's discarding was too much for most decks, it had a niche. A few decks, like those relying on Naganadel for Energy acceleration, could take advantage of Sightseer's effect by dumping a bunch of Energy from the hand into the discard pile and drawing a little at the same time. Mewtwo & Mew-GX could get the same effect but with Pokemon-GX. A lot of decks still went the more traditional route instead of using this, as it wasn't like game lacked ways to discard. There were things like Ultra Ball and Dedenne-GX, after all. Sightseer did provide another way to get through cards, though, so a few people saw success with it.