"Jar of Greed" lets you draw a card when you activate it. Some Duelists ask, what is the point? You have to draw and play a card ("Jar of Greed") to draw another card. So there is no gain.
If you have a Deck that is exactly 40 cards, you begin the game with 5 of them. So that leaves 35 cards in your Deck. Each time you draw a card, you have a chance of drawing a specific card. If you can put 3 copies of "Jar of Greed" in the Deck, it really shrinks the Deck size from 40 cards to 37 cards. After drawing your starting 5, that leaves just 32 cards left in the deck.
Because you cannot make a Deck smaller than 40 cards, effects like "Jar of Greed" are ways to make it like you do have a smaller Deck. A 37 card deck, which is what 3 copies of "Jar of Greed" gives you, gives you a better chance to draw your favorite cards than in a 40 card Deck. This strategy is called "Deck Shrinking".
"Jar of Greed" also makes a great bluff card. Who knows what your opponent will think it is?