A selection may contain more than one object. Sometimes, you have spent some effort selecting several objects, and then you want to select a few other objects as well. Can you do that without starting the selection all over again?
Yes. To add another object to the current selection, 
-click it. If, however, you -click an already selected object, it will be removed from selection. In other words, this shortcut works as a toggle that reverses the selected status of the object on which you -click.