The blocking off, of seats is to ensure the aircraft remains in trim. For flights that have allocated seating, check-in staff would be restricted to the number passengers that can sit in different areas of the aircraft to enure it remains in trim.
Once airbourne you would be free to move seats but may have to return to your allocated seat for landing.
It's always going to be on flights that have a free seating policy that your goin to see the blocking off, of seats if the flights isn't full.