There are a number of possible answers to this question. Given the way you have worded the question, it appears that you are probably expected to give one specific answer, probably from your book. Therefore, I suggest that you check your book in addition to reading this answer.
One possible answer is that the Louisiana Purchase increased the size of the United States tremendously. In fact, the land area of the US was roughly doubled by the purchase. This was significant because it made the US so much bigger and included so much more land that was full of resources within the country.
A second possible answer is that it helped make the US more secure. Before the purchase, a foreign country could conceivably have colonized North America as far east as the Mississippi River. This could have put a hostile country on what is now the border of Illinois or Kentucky. By purchasing Louisiana, the US ensured that any foreign presence on the North American continent would be farther away from the main US population centers.
A final possible answer is that the purchase made the interior of the US much more economically viable by assuring US access to the Mississippi River. In those days, it was very hard to move goods by land so most long-distance transportation was by water. The farmers of the interior had to rely on water transport to get their goods to market. If the US did not own the Mississippi River, a foreign country could block Americans from using that river, thus cutting the interior off from the ocean. The Louisiana Purchase, then, was important because it secured the economic viability of the American interior.
One of these answers is most likely the one you are asked to provide. I suggest that you consult your book to find out which one your teacher wants to see.