Suppose you have a diagram that has been arranged with hierarchic layout. Then you add a new node and connect it to another node. When you now run hierarchic layout again, the whole diagram will change. Even if you run hierarchic layout with option "Selected Elements Incrementally" (and only the new node selected), the coordinates of all elements will change (even though their relative positions will be kept).
If you run "Selection (Partial)" after inserting and selecting the new node, the coordinates of unselected elements will not change at all. Only the selected new node is moved.