The main reason for this system is due to the London 2012 Olympics when players tried to avoid better pairing in the second round or quarter final matches. So most of them fake their matches in order to loose a match to get a better draw.
After this fiasco, those player involved in such practice was given the boot in the Olympics. That’s the reason why Rio 2016 uses the draw lots system. You will be seeded if you are rank 1st to 4th. Other teams will need to draw lots regardless who finishes first in the group.