The purpose of the VLOS rule is based on the fact that while flying FPV (using a tablet screen alone), you can only see the airspace directly in front of the drone. You cannot see other aircraft that may be approaching from above, below or from a different direction. You also cannot see obstacles that may be in the vicinity if your view is only forward. So the FAA requires the use of spotters that can see then entire airspace around the drone in situations when the pilot cannot.
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