It's important to remember that at this time in US history, the 1830s, the infrastructure of the country was very primitive and inefficient. The National Road, or as it was also known, the Cumberland Road, did in one fell swoop what the system of interstate highways did later in the 1950s.
The road ran through five states, from Maryland to Illinois, joined 620 miles of territory east to west and not only opened up the frontier to business, settlement and economic growth, but made for a much more efficient system of transportation and distribution of finished goods moving west and agricultural products moving east.
1) The road was heavily used for shipping of wheat and corn from the west to eastern markets, allowing those cities to support larger populations, and making farms in the west more profitable.
2) Living in rural western and even frontier towns became more possible, as the finished goods factories put out in the east could be shipped more easily and cheaply westward, making those companies more profitable and living in the west more attractive.
3) The National Road led to the growth of towns all along its route, just as the railroad later would farther west, as it became the most efficient place to locate a city, and the most convenient place to live. This solidified US control over new territories, and attracted immigrants and migrants.