The point is to help direct air flow where it's most needed.

On a blazing hot day, if you are driving alone, do you need cool air blowing onto the empty passenger seat? So instead, close off those 1 or 2 vents, and increase the cold air blowing on you.

Or if 2 people are in the car, but they have different tolerances to the cold air blowing on them, one can open the vents all the way, while the other reduces or cuts off the air flow.