The idea of Manifest Destiny, broadly defined, has both postive and negative impacts on the US. In general, it has made us a more generous country, but it has also caused us to be arrogant and overly aggressive.
Because we think that we should spread our democratic system across the world, we are often ready to help other nations. We send the Peace Corps out to help people in other countries improve their lives. We have programs to help do things like setting up legal systems in newly independent countries. We are essentially always ready to help other countries in the hope that they will come to see that our ways are beneficial.
At the same time, however, we tend to overreach due to our belief in Manifest Destiny. We think that we can go to other countries and easily induce them to see the wisdom of being like us. We have ended up in long and fruitless wars in places like Vietnam and Iraq because of this belief.
Our belief in Manifest Destiny has made us generous and willing to help, but it has gotten us in trouble because it makes us think that we are capable of doing more for the rest of the world than we really are.