Why was the Louisiana Purchase important to the US?

The Louisiana Purchase totally changed the future of the US. First, it was an example of a president going his own way without consulting Congress. Technically, he was to ask permission to spend government money on such an endeavor, but he didn't. And, second, lucky for him, it worked out well. It doubled the territory of the US, and as often happens, with more land comes more power, resources, and opportunities. Without this development, the US would have been contained into the east coast and couldn't have expanded into the country it is today.
He sent people out to explore the land in order to glean information about its opportunities and available resources. And, he wanted to find an all water route to the Pacific, and he wanted to develop good relationships with Native Americans in order to trade with them, assimilate them into US customs, and gain more land. Once the land was explored, it could be settled in and used. In the end, his explorers (Lewis and Clark and company) were successful, and they brought back invaluable information about the land to the West. Sadly, they couldn't find that all water route to the Pacific, but their trip wasn't in vain. This instigated the desire for settlers to travel west and to build new communities. It was part of the basis of the American dream: start from the bottom and build your way to success.

It was important primarily because it more than doubled the size of the United States. This huge territorial expansion meant that the country now stretched from the Rocky Mountains in the west to the Mississippi River in the east, and from the Canadian border in the north to the Gulf of Mexico in the south. Fifteen new states were created out of this new territory, providing the United States with additional resources of land and raw materials that would greatly add to the nation's economic prosperity.
The Louisiana Purchase was also important in political terms. The vast, wide open spaces that it suddenly opened up gave more Americans the opportunity to live out the Jeffersonian ideal of a nation consisting of independent small landholders, whose close connection with the soil would give them a stake in the country's stability, thus making them the protectors and defenders of republican liberty. Whatever the importance of the Louisiana Purchase, there can be little doubt that it represented one of the greatest triumphs of Jefferson's presidency.