It makes the trailer and tow Vehicle more stabile. It effectively makes the wheel base longer on the towing vehicle, so it is not affected by cross winds as much. I think some of the comments here are based on ignorance.
Not only is it more stabile, it preserves the trucks full payload capacity for cargo.
Only criticism of this particular design I see is the missed the opportunity to add an extra set of braking wheels, which would have shorten stopping distance.
I never towed this exact set up, but I have towed a wagon. Anyone who has towed a wagon , will tell it is a much easier tow, and better ride. The only disadvantage is backing up.