From my experience......

The roller doesn't actually change the temperature but what it does is only select the ratio of air that you like.

Climate control works by cooling air to minimum temperature and then introducing a little or a lot of trim hot air to bring it up to the temperature that you require.

The roller basically does the same so you'll find if you have the interior temperature selector set to 18 degrees and have the roller set to full hot the airflow from the middle vent will be just above zero however if you then set it to full cold you'll find it will blast out of that vent and vice versa.

The idea of it is to set a temperature that is comfortable for your face and regardless of what your climate control is doing the temperature on your face will always be pleasant.