Terrorists sometimes try to disrupt the economies of the rich countries of the world. There are three main reasons why they might want to do that.
First, they might want to disrupt rich world economies simply because they want to hurt the rich world. Terrorists tend to feel that the US and other rich countries can hurt their (the terrorists’) own kind of people while they cannot really hit back. By hurting the US economy, they are getting revenge and inflicting pain. That is important for its own sake.
Second, they might want to reduce the standing of the United States (or other rich countries) in the world. If the terrorists can hurt the US economy, they are demonstrating that the US is not really that strong. The terrorists are demonstrating that the US is not able to prevent them from hurting it in important ways. This weakens the US and enhances the terrorists’ image.
Finally, by hurting the US economy, the terrorists can hope to reduce its ability to influence the rest of the world. If the terrorists can hurt the US economy, they can hope that Americans will become more preoccupied with economic issues and will forget about trying to exert influence in the Middle East or other such places.