It depends on the goal of the DM. Inspiring dread in characters is easy, but in players is hard. A good roleplayer won't need the jar, as the in-game description should be sufficient to bring dread to the character. However, since the player is never in danger, it's hard for many players to feel the sense of dread that their character probably does. This is mostly found if you try to run a horror based adventure/campaign.You've forgotten the most basic rule of role-playing - that players can only act on information available to them. Your little show should be meaningless, since any half-way decent player will disregard it as irrelevant.
If the DM wants to bring a sense of dread to the players, normally something out of game needs to be done. Many DMs use various techniques to create an ambiance (using lighting, mood music, etc.), and the jar would be a useful tool along the same vein.