It's a hub for NPC that have died in the story. You can go there and talk to them to get exclusive missions to do back in the real world.
However, you only ever get 4 quests from them, and after you do the quest, the corresponding npc will leave the Interim. Because of this, I think the Interim is under utilized, but it is what it is.
Slayer of Demons,Inquisitor,Ylissean Tactician,Kindred Hunter,Gray Warden,Dark Lord,Good Hunter,Chosen Undead,Nerevarine,Exile,Mugunghwa Investigator,Tremere Anarch,ESPer,Squad 7 Hero,Nohrian Prince,Arisen,God Eater,YoRHa Android,Tenno,Hope's Peak Dropout