What is the purpose of PLACEHOLDER_ID ?

[mattbal]

I'm going through the MultipleContainers example and I'm confused about the purpose of PLACEHOLDER_ID. What is its purpose? Why do we need it? Is it in case the user drops an item on the Add Column button?

const PLACEHOLDER_ID = 'placeholder';

// ... omit

<DndContext 
  onDragEnd={({active, over}) => {
    // ... omit
    if (overId === PLACEHOLDER_ID) {
          const newContainerId = getNextContainerId();

          unstable_batchedUpdates(() => {
            setContainers((containers) => [...containers, newContainerId]);
            setItems((items) => ({
              ...items,
              [activeContainer]: items[activeContainer].filter(
                (id) => id !== activeId
              ),
              [newContainerId]: [active.id],
            }));
            setActiveId(null);
          });
          return;
        }
  }
>

// ... omit

<SortableContext
  items={[...containers, PLACEHOLDER_ID]}
  strategy={
    vertical
      ? verticalListSortingStrategy
      : horizontalListSortingStrategy
  }
>

// ... omit

{minimal ? undefined : (
  <DroppableContainer
    id={PLACEHOLDER_ID}
    disabled={isSortingContainer}
    items={empty}
    onClick={handleAddColumn}
    placeholder
  >
    + Add column
  </DroppableContainer>
)}