Often in a complex layout we have to select many graphic in same time to apply some modification on them (moving, deleting...). Very often i do a mistake and select an other item which i shouldn't. Or it's just much faster to make a rectangle selection and unselect some items individually.
If i'm not mistaking, there is no possibility to unselect it individually by clicking on it again. Then i have to unselect all and start again, which is quite painful.