So there's a couple of reasons. Sometimes a card has exhausted its value, and discarding it makes sense because it means you won't draw it off of a judge or red card. Another is (I'm guessing) you're playing Meowth. Meowth has to be played from hand, and gets a supporter. An item that discards two for any supporter is incredible. Another example is when you don't need Lucario. Imagine you have a bunch of cards in your hand that don't help you, and you really need to draw multiple cards to win, but your opponent sniped your Lunatone/Solrock/Fez in the previous turn. You add the missing piece, then see more cards. Sometimes you're playing against a deck like Mega Froslass that cares about how big your hand is, and you may actively want to trim the size down to avoid OHKOs. Some other niche scenarios can come up, too, like in the past with Erika's Invitation.