The nature of manifest destiny is fairly clear. The purpose, to the extent that there was one, is less clear.
Manifest destiny was an idea. It was an idea that held that the United States had been destined by God to spread itself across the North American continent. It is not clear whether most proponents of manifest destiny had a specific set of ideal boundaries for the US in mind. For example, it is not clear if they thought the US should take Canada or all of Mexico. Even so, they did believe that the US deserved to have more land and that God wanted this.
Manifest destiny held that Americans were superior to other peoples. It held that the American political system was superior to that of other countries. It held that American Protestant religion was superior to Catholicism. It held that Americans were racially superior to people such as the Spanish and the Indians. It held that America’s culture was superior to that of other countries.
As to the purpose of this idea, it is hard to say. If a person states a fact that they believe, there is not necessarily any purpose to that statement. However, we could speculate that the main purpose of the idea was to justify American expansion.