Making a selection is not necessarily the end of your work. You can add to the selection, subtract from the selection, expand and contract the selection, and more.
After making an initial selection, you can add to it. To see how this option works, open a new 500×500, 72dpi image with the background set to white. Then follow these steps:
Choose the Selection tool and, in the Tool Options window, set the Selection Type to Rectangle.
Draw a rectangular selection in the upper-left corner of the image.
Place the mouse in the lower-right corner of your selection (not too near the corner, though) and, while holding down the Shift key, draw another rectangular selection. You should now have an area selected ...