Why did Jefferson feel that it was important for the United States to control the Mississippi River?

Thomas Jefferson understood how important control of the Mississippi River was for both economic development of the west and the race to expand the United States’ influence beyond the colonies. The United States was one of several competing countries who desired control of the river. The Mississippi River provided a lifeblood to agriculture along its banks. Not only did the river prove to be a fertile basin for farmers, but the river also provided a reliable transportation system for the young country’s farmers to move their goods and services.
Westward exploration and expansion was also a result of control of the Mississippi River. Jefferson’s Louisiana Purchase in 1803 from France for roughly $15 million fortified US ability to reach the Pacific Ocean and provide a protective buffer against potential enemies. Although there were concerns about the US overextending itself with the purchase, Jefferson correctly surmised that the principles used to govern several colonies could also govern many states.
https://www.monticello.org/site/jefferson/louisiana-purchase
https://www.nwcouncil.org/history/Jefferson

There were several reasons why Thomas Jefferson believed it was important for the United States to control the Mississippi River. The Mississippi River was very important to western farmers. They used the river to transport their crops so they could get their crops to markets where they could be sold. Spain had previously controlled the port of New Orleans. Eventually, Spain cut off the right of the United States to deposit or store goods at New Orleans. This could be a crippling blow not only to the western farmers but also to the economy of the United States. When President Jefferson learned that Spain had transferred control of Louisiana, including the port of New Orleans, to France in 1802, he decided to try to make a deal with France that would give the United States control over New Orleans and the area around it.
President Jefferson wanted to buy New Orleans and West Florida for ten million dollars. When Napoleon countered with an offer to buy the entire Louisiana Territory for fifteen million dollars, Jefferson had to be convinced that this was constitutional. By purchasing the entire Louisiana Territory, the United States controlled the Mississippi River and the port of New Orleans. This would allow farmers to ship their products on the Mississippi River to the port. Other products could also enter the country through New Orleans. This action not only helped western farmers and the American economy, it also showed the people living in the West that the government cared about them, their interests, and their welfare.
https://billofrightsinstitute.org/educate/educator-resources/teaching-with-current-events/

https://www.lehrmaninstitute.org/history/louisiana-purchase.html