And for your further education: The scale tool will work well in a simple example like this. However, as its name says, it scales the entire object you have selected, it does not simply move an edge. For a plain rectangle, there is no meaningful difference. But for a more complex shape, every aspect of the entire shape will be scaled proportionally. For example, if your rectangle was the outline of a room and you had an inner rectangle for the walls, the thickness of the walls will also be affected.
