I didn't need the packers on my 4Runner, but I have used them before on other vehicles. They are primarily for correcting height differences from left to right, but many people like to use them on both sides for increased lift overall.
I helped a friend install an OME lift on his 90 series Prado and we used packers on one side because he had a heap of touring gear permanently setup in the back and it wasn't evenly distributed weight wise, hence on flat ground it had a slight lean to one side.