Why New Orleans (victim of hurricane Katrina) is vital

The Louisiana Purchase (this is back in the 19th century if I am correct) was important because it gave the US the port of New Orleans, the port at the mouth of the Mississippi. Even today the New York Times in an article titled. "Across the US, outrage at response" said, All the grain that's grown in Iowa and Illinois, and the huge industrial output of the Midwest has to come down the Mississippi River, and there has to be a port to handle it, to keep a functioning economy in the United States of America."