In 1969, when NASA landed mankind on the Moon for the very first time, it was no easy task; and NASA wasn't even sure that they could accomplish it. Nonetheless, they tried their hardest, and it wasn't without its failures, but they finally made it.
Back in the day when we didn't have all of the fancy computers and computing power that we have today to help us model everything out, all of these calculations had to be done by hand by rocket science experts.
By putting a type of orbiter around orbit of the Moon, the orbiter could then drop a lander on the Moon that contained American astronauts, and that lander could then blast off from the Moon's surface to get back to the lander where it would then descent back to Earth.
When it was said that it was ‘one small step for man, one giant leap for mankind,' Neil Armstrong was absolutely right. Mankind has visited the Moon over and over again, and now NASA wants to try to put a man on Mars.