This mission was the first flight test of the Lunar Module, the third critical piece of Apollo hardware. During ten days in earth orbit, the crew undocked, maneuvered and docked the LM and the Command Modules, simulating as closely as possible the conditions that would be encountered when men finally would land on the moon itself.
This mission also tested the Apollo spacesuit, the first to carry self-contained life support rather than being dependent on an umbilical connection.
This was the first Apollo mission where the astronauts were granted the honor, often exercised rather whimsically, of naming their spacecraft. The Apollo 9 astronauts named the Command Module "Gumdrop" and the Lunar Module "Spider."
After Apollo 8, Apollo 9 was a bit of an anticlimax, but it was another absolutely necessary step that we needed to take if we were going to place a man onto the moon.