If you enlarge the photo of the Fit(mine is white)you will notice that the roof line and the rear spoiler are at about 15 to 20 degrees to each other.You are not streamlining you are moving the amount of pressure around.
The spoiler keeps the air moving in a slightly upward angle(little drag) but at the same time speeds up the air from under and sides said trailer to fill the gap reducing drag and lift at the same time.You are trying to make the air going under the car or trailer travel as far and fast as the air going up and over to equalize the pressures as much as possible.The very little drag added by the spoiler.
Without the spoiler the air from on top has to fill the big gap left behind witch causes a lot of drag.The spoiler makes the air from around the rest of the car,trailer or what ever help fill the gap causing less drag and lift.