The Inbox Folder is where new Epics, Stories, Issues or Ideas, all depending on where the Inbox Folder is located, are put when created from the main menu or in a Roadmap Lane. It is of course possible to create new items beneath folders or Epics in the Storymap or Outline, but if you don’t know yet where they should be placed (or you are a Limited User who is not allowed to do so) – putting these in the Inbox is a good choice. Furthermore, items delegated to a circle will be placed in the Inbox first as they await to be placed in the appropriate areas of the Storymap tree.
The intention of the Inbox is (as the name implies) that it is regularly emptied – however, it is not mandatory. Its items should be placed where they functionally belong in the Storymap to obtain a better overview instead of just piling up in the Inbox.