The inbox is your starting place for all new stuff that doesn’t yet have a Plan. Once a task gets a Plan, it automatically moves out of the inbox.
Stuff is setup such that tasks cannot exist in the Inbox with a Plan. Once a task is given a Plan, it automatically moves out of the inbox and is in “no list” by default. By the same token, if a task is moved to the Inbox, it loses it’s Plan.
The theory of productivity built into Stuff is that tasks in the inbox should be prioritized as needing to be put elsewhere. That could mean adding tags to them, moving them to an appropriate list or space, or giving them a Plan or Deadline.