The Inbox is where new stories (and issues/ideas or impediments/improvements) are put when created from the main menu. It is of course possible to create new stories beneath epics in the Storymap or Outline, but if you don’t know yet where the story should be placed (or you are a Limited User who is not allowed to do so) – putting new stories in the Inbox is a good choice. Furthermore, it a story is delegated to a circle, it will be placed in the Inbox.
The intention of the Inbox is (as the name implies) that it is regularly emptied – however, it is not mandatory. Stories should be placed where they functionally belong in the Storymap the obtain a better overview instead of just piling up in the Inbox.