Icons of any type are to compensate for the poor relationship between what you can see on a monitor vs. what you can see in real life. It has been stated many times that the default zoom is approximately 1 to 1, whereas what you see without zoom is purposefully made smaller/farther away to help simulate a modicum of peripheral vision in the limited field of view the monitor provides. Likewise without the icons the lack of and/or distorted visual cues you receive from a 2D monitor would make it very difficult to judge even whether you are closing or not closing the distance to an enemy -- which in real life would be fairly easy to distinguish.
If you've ever flown in a low flying plane or taken a helicopter ride, you will note that ground objects are actually quite easy to pick out. Take any commercial flight and up to a quite significant altitude you can still pick out individual buildings and cars driving on the interstates. So from a plane flying at 3000 feet above ground level, picking out something the size of a tank in the surrounding fields would also be comparatively easy, especially if that tank was moving. So, we have icons on the GVs, too.
