It about doubled the amount of territory owned by the fledgling U.S., and for a very reasonable cost. It also provided the U.S. with some badly needed security.
It's the equivalent of someone wanting to buy a house, but getting a large city for the same price.
Spain had control of most of the territory, including New Orleans. Revoking treaties with the U.S., they denied access to the city, which controlled traffic coming and going from the Mississippi River.
France eventually took back control of the area, which caused some concern among Americans that she may try to invade U.S. Territory.
While Napoleon wanted to create a North American French empire, what he got was a Haitian revolt. The U.S. remained neutral during this, but allowed the smuggling of supplies to the Haitian rebels.
With diminished revenue from the Caribbean, France really had no need of Louisiana, and it was afraid of a British or even a U.S. invasion of the territory.
The U.S., for its part, was afraid of French aggression. So Jefferson began negotiations to buy Louisiana. Eventually, what he got was the entire French possession.
The purchase encompasses all or part of 15 states, and cost 50 million Francs (about 11 million dollars). It included the forgiveness of 18 million Francs (3 million dollars) in debt France owed the U.S. for a total of about 15 million dollars, or about 4 cents an acre. In 2013 dollars, that's about 240 million dollars, less than 42 cents an acre.