First of all, please note that your textbook may give certain factors that you are supposed to give back as an answer to this question. This answer is not based on your textbook and may not be "right" when compared to that book.
There were a couple of different kinds of reasons why Jefferson wanted the expedition to happen.
- Economic reasons. He wanted to be able to trade with Indian tribes. Perhaps more importantly, he was hoping to find a water route to the Pacific Ocean so that trade from East to West could happen more easily.
- Intellectual reasons. Jefferson was a scientist of sorts who was very interested in a variety of things. One of the things he wanted to know was more or less everything about the rest of the continent. He wanted to know about the people and the plants and the animals and the geography of the rest of this "unexplored" continent.