This may look useless to some people however it can serve, in some cases, if your opponent forces the discard of all cards in your hand, to draw a card immediately and not stay with no cards in your hand or to reduce the number of cards in an "Exodia" Deck. "Jar of Greed" is an important and useful card to reduce the number of cards in your deck. The fact is that the fewer cards you have in your deck, the greater chance you will draw your crucial cards. Other cards, such as "Magical Mallet" and "Reload", have similar effects.
You can bluff with this card, then, when your opponent activates a card to destroy it like "Mystical Space Typhoon", you can chain it. You get a replacement card and your opponent has wasted theirs.
After using "A Cat of Ill Omen" to place a choice trap on top of your Deck, you can use "Jar of Greed" to draw it right away, or to make sure you draw it before your opponent can force you to shuffle or destroy the top card. You can also use "Jar of Greed" to recover lost tempo if your opponent uses "Back to Square One" or a similar effect on you.
As a Deck thinner, only include this card in decks with only 40 cards. Otherwise it'd be entirely easier to simply not include this card (and thus one less card in the Deck).
Use this card when you have "Muka Muka" or "Tragoedia" on the field when your opponent attacks "Tragoedia" or "muka muka" and the monster's ATK is less than 300 or 600 more ATK than your face-up "Tragoedia" or "muka muka" respectively.
In a stall deck you can consider "Jar Turtle". If the turtle is on the field by chance you will get an extra card and its defense is really helpful.