Suppose that we want to change the reporting of "take" so that the player is always told something like "You take the book from the shelf." or "You pick up the toy from the ground." In order to generate these reports, we will need to know where the object started, even though by the time we are printing the output, the object will have moved.
The previous locale could in theory be either a thing or a room, so we make it "an object" -- that is, the most generic possible kind, to which both things and rooms belong. Now we record what the previous locale is at the beginning of each taking action:
Test me with "get all".
You can see a desk (on which are a pill-counter (in which are some Vicodin), a prescription and a computer) and a trash can (in which is an empty box) here.
> get all
trash can: You pick up the trash can from the ground.
pill-counter: You take the pill-counter from the desk.
Vicodin: You take the Vicodin from the pill-counter.
prescription: You take the prescription from the desk.
empty box: You take the empty box from the trash can.