Why did President Franklin Roosevelt want to offer help to the Allies?
He wanted the US to be involved in European and global affairs.
FDR was not an isolationist and thought that America had a role to play on the world scene. Furthermore, he believed that German dictator Adolf Hitler was a threat to his world vision of peace and prosperity. Before Pearl Harbor, it was impossible for him to convince Congress of his ideals (as the Congressmen were isolationists), despite his use of massive propaganda by hiring filmmakers, like Capra, to promote anti-Hitler movies.
WWI had made America dominant over Europe financially and Hitler clearly threathened that hegemony. The 1944 Bretton Woods meant that the US dollar was the World Reserve Currency and the 1947 Marshall Plan implied that Western Europe had to rely on the USA to finance its rebuilding.
After the war broke out, FDR clearly intended to join the war and promise the British monarch that he would do so on their side. FDR created invented the paradigm of the Welfare-Warfare state, as he promoted state intervention in the economy at home and massive military intervention abroad.