The National impact was huge. It more than doubled the size of the United States. It gave access to the Mississippi River. This allowed for an easier way to ship goods back and forth from the Ohio River Valley and other interior lands.
It gave us access to the port of New Orleans. This gave us another port and markets to sell goods.
Internationally this allowed us access to the goods and markets of New Orleans. The money for the purchase will go to fund Napoleon's war effort to take control of all of Europe.