The Preference number tilts where your dice roll is likely to land between the four possible outcomes. It is a bit like the Number in Lasers&Feelings in that respect. You roll the d6 and add your Preference, and use the result to determine whether you succeeded on the requested ability:
If I pick a -2 Preference, then my range of possible results is -1 to 4. That means I have a 5/6 chance of success on a Rational roll, but only 1/6 chance of success on a Fantastical roll.
If I pick a +1 Preference, then my range of possible results would be 2 to 7. In that case, I only have a 1/3 chance of being Rational, I have a 1/2 chance of being Physical, 5/6 chance of being Emotional and 2/3 chance of being Fantastical.