You get priority. That's what the "preferred" in preferred seating is about.
But I would agree that this ST benefit has become less and less relevant since more and more airlines now allow you to choose your seat at the time of booking, or via the internet before check-in, or at least via internet check-in. This benefit comes from the pre-internet days and used to be extremely useful. Nowadays it really depends on the airlines's general seat policy whether this makes a big difference. It in any case it is still very useful when you have short-notice bookings or flight changes and hate middle seats, as I do, or suddenly have a tight connection and need a seat in the frot to make your next flight.