There are cards, that when you first look at them, you know they're cool. Others you know aren't. And still others you're not quite sure until you try them. I daresay for many, Elven-light fell into that last category. ("Play it only from my discard pile? Then how does it get there?") The pool of cards that benefit you by discarding other cards from your hand has grown, to the point we now have Silver Harp, where you prevent it from happening.
For any discard deck (Arwen, Círdan, and Erestor come to mind as Noldors love to discard; it's in Tolkien's official LotR compendium) the power of Elven-light becomes apparent when you discard it, because not only do you get the benefit of the discard effect (as in Arwen's case) but if you have enough resources, you can spend 1 to draw a new card... and get Elven-light back in hand. The last part is very significant (as opposed to removing it from the game) as you now get to use it again. And again, and again, and again.
For instance, envision you have Protector of Lórien or Elven Spear attached. If you have 2-3 spirit resources to spare, then with only 1 Elven-light, you could discard it 3 times and have 2-3 new cards drawn from your deck. Of course, you don't want to squander resources, so it's not always best to use it right away, but that's another great aspect, as it'll sit in your discards until you're ready to play it.
If you haven't tried a discard-centric deck, definitely do so as it feels different and fun. It'll make you appreciate this card, as it turns resources into cards, while either fueling your best discard effects, or serving as excellent fodder when you're forced to discard, such as with Daeron's Runes or Círdan.