Some spoilers are very well engineered that they will increase the downforce and decrease the drag.
I have seen such spoilers from TRD, Mazdaspeed, Mugen, etc.
If you add spoiler, it changes the airflow of the back of the car.
My amateur guess is that it changes the slipstream so that low pressure air isn't dragging the car backwards anymore.
As you can see, these vehicles create a lot of drag
Solar cars like Nissan Sun Favor, Toyota Rara10, Honda Dream had drag coefficient of 0.10 to 0.11