Saving a selection allows you to edit a selected area of a photo at a later time. You can work on other parts of the photo before loading the saved selection.
Intersect With Selection
Replaces the saved selection with the intersection between the current selection and the saved selection.
You can also modify a saved selection by loading it and using selection tools to add to it (Shift-drag) or subtract from it (Alt-drag in Windows, Option-drag in Mac). (See Add to or subtract from a selection.)