He came to power because he was a great orator and convinced the people that they were doing the right thing. He promised change and replaced the weak government they had before.
The main impact of Shays' Rebellion on the formation of the Constitution was a federal government that was more powerful and capable of keeping the states in order. The rebellion made delegates fearful of the possibility of the entire country collapsing into anarchy in the absence of a powerful central government.
The most notable is the atomic bomb. Roosevelt became aware that the Nazis were trying to develop one, so he started the "Manhattan Project" which was a group of elite scientists tasked with developing their own atomic bomb. They eventually succeeded, and in 1945, we dropped the bomb on Japan.
