While the Moon landing missions garner the most fame, the others were essential to those ultimate successes. Apollo 8 first took humans to the Moon, and Apollo 10 served as the dress rehearsal for the first landing. But in between was Apollo 9, which was needed to prove the capabilities of the complete Apollo spacecraft. They successfully demonstrated module docking and extraction, proving Apollo was capable of a successful landing and return. It proved that Apollo was capable of a complete, successful round-trip involving landing on the Moon.

Source: Forbes March 04, 2019 06:56 UTC