Why did many Americans believe it was important to expand u. s territory

To increase population.

Explanation:

During the time between 1803 & 1840 the Louisiana Purchase had taken place, it had doubled the size of the country which increased the population of the area. Therefore they had decided to expand even more over time for more population, it also is now referred to be called Manifest Destiny

United States believed that expanding U. S. territory was their rightful destiny also called Manifest Destiny.