Jack and his group do not take the conch from Piggy. In fact, Piggy is holding the conch when he dies: "The rock struck Piggy a glancing blow from chin to knee; the conch exploded into a thousand white fragments and ceased to exist."
When Jack and his group first raid Ralph's camp earlier in Chapter 9, they take fire. In the subsequent chapter (The Shell and the Glasses), they take Piggy's glasses. It's important to note that Jack does not want or need Piggy's conch. The conch represents a democratic order, a type of organization in which anyone can have a say, a voice. Jack rules his tribe as a dictator. He makes all the rules. Unlike Ralph, he does not seek input from the group when decisions are made. During this raid, Piggy mistakenly believes that Jack wants the conch. However, Jack really wants Piggy's glasses. When he has these in his possession, "he was the chief now in truth." His group is completely independent of Ralph's because now he can make fire on his own.
Piggy's glasses do to some extent symbolize intelligence and rational thinking. But when Jack has them, they represent a means to create fire for cooking, and later in Chapter 12 fire becomes a weapon. Jack rules by whim, not rationality, so when Jack holds the glasses, their previous association with intelligence is lost. Now they represent a means to satisfy more primal instincts. This raid is particularly important in that Jack assumes complete dominance on the island. With glasses in hand, he does not need Ralph or Piggy. He has the island under his control. He can have Winfred tortured at will; he can hunt as he pleases; he can have feasts; he can kill whomever he deems his enemy.