This is the place where they drink from a fountain containing a potion that restores memories. Whitelighters come here during retirement to retrieve their lost memories of their mortal lives. A code needs to be put in magically in order to open the vault.
In "The Source Awakens", Harry Greenwood and Mel Vera met in Seattle in Macy Vaughn's third altered reality and their brains flooded with memories from both their original reality and the new one, prompting the two of them to orb to the vault. In order to get through the code faster, Harry had Mel use her bracelet to speed up the process. The two then drank from the fountain to restore their memories before orbing to Maggie to restore her memories with the same potion.