There is some amount of debate about the motives of the Framers of the Constitution. There are people who have argued that the Framers were motivated by their own economic interests. These people (most famously Charles Beard) believed that the Framers were economic elites who wanted a new constitution that would do more to protect their wealth.
However, most scholars feel that the Framers were motivated by a desire to create a stronger country that would be of benefit to everyone. The Framers felt that the country under the Articles of Confederation was too weak and that it would never be able to become strong militarily or economically. This, they felt, would be bad for all Americans so they wanted to create a new constitution that would fix these problems.
The major impact of the Constitution was to create a more unified country. Before, the states had practically been independent little countries. After, the national government had much more power. As the country became more unified and more centralized, it was able to grow stronger.