The term Manifest Destiny was originated in 1845 by John O' Sullivan as an idea that the U.S was destined to spread from coast to coast in a westward direction.
Americans said they were to have a superior form of government, a superior culture, and a superior religion. Because of those reasons, they thought they were destined by god to expand their territory.
The idea behind Manifest Destiny was that it was our duty or destiny to expand from the Atlantic Ocean to the Pacific Ocean. The idea of Manifest Destiny influenced the America's westward migration because it made migration more likely.
Manifest Destiny was a good thing because it allowed a nation to be able to expand their territory. Because of Manifest Destiny, land associated with the nation expanded the country's size by over 60%.
The bigger the population size got, the more food you would need to have. Manifest Destiny created more land that could be dedicated to crop development. It brought more land for farming and improvement
Lastly, because of Manifest Destiny, there would be more routes for trade and barter. It opened more doors to trading with other countries.