A US strike would cause immense damage to Iran’s nuclear program. It is unlikely that Iran has significant operational nuclear facilities that America doesn’t know about. The United States could destroy Iran’s known facilities.
While US government officials have said that an Israeli strike would only set back Iran’s nuclear program by one to three years, the US, with its superior capabilities, would impose a greater delay of three to 10 years. This purchases much time for diplomacy or other events that could result in permanently keeping Tehran from the bomb.