The Inbox is where new Epics, Stories, Issues or Ideas are put when created from the main menu or in a Roadmap Lane. Stories and Epics are put in the Functional Inbox and Issues and Ideas are put in the Issues and Ideas Inbox.
It is of course, possible to create new items below folders or Epics in the Storymap or Outline, but if you don’t know yet where they should be placed – 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. One way of doing this is to show the inbox items in the left panel in Storymap and drag them to their proper place.