For example, I want to send an stationed Overlord by base, inside. Instead of moving the mouse to the minimap, clicking it, selecting the Overlord, and giving him the move command, this would only require holding the key, making a small box to select the green dot at the location to select him and giving the move command to the base. Of course many actions are done on minimap already, but this would be faster and more precise.
Also, for example if I am situation that I need an Overseer asap while not having it hotkeyed, it would be possible to use the new full army select hotkey, and then alt-v to highlight just the Overseer and move him. Now, the select all army hotkey is basically useless.
I feel that this would be used by pros as well as by beginners -- I am quite sure that there are many players not familiar with maps and the minimap is not sufficient for orientation.