On March 7, 1969 during Apollo 9, the first manned flight of a Lunar Module took place. After a few days docked in earth orbit, Spider, separated from the command module and for the first time Astronauts flew in a spacecraft that could not get them home. The photo shows them after separation. This mission is overlooked because they didn't go to the moon but they tested nearly everything required for moon landing. The first testing of the PLSS backpacks that they would wear on the moon. The first docking/undocking/redocking of a LM to the CSM. Testing of the descent/ascent engines of a LM. Being able to maneuver to different orbits depending on who needed to be the passive or aggressor in a docking operation.
One very important test that took place wouldn't be utilizeed again until 4 missions later for Apollo 13. They tested the ability to maneuver the joint spacecrafts using only the LM engines, something later used to save Apollo 13. Contingencies. You never know when you will need them.