What was the purpose of the Louisiana purchase?

The purpose of the Louisiana Purchase was to give the United States the port of New Orleans and thus control of the Mississippi river. Any other power in control of New Orleans would have been a natural enemy of the US because it could choke off all commerce from the western slopes of the Alleghenies, which went by river boat and flat boat to New Orleans and from there to the East Coast and the rest of the world.

As it turned out, the Purchase also nearly doubled the size of the United States and this side effect, thought to be a waste at the time, may have been the primary long-term benefit of the transaction.