Even though I have flown over 2 m miles, I still pay attention to the safety briefing. These briefings are to orient the pax's mind set. It is the same reason to look for fire exits each time I stay at a hotel as well as reviewing the opening mechanism of the exits each I sit in the exit row. It is the mindfulness that help a person react correctly in time of emergency. As FINKFACE mentioned, muscle memory and person habits can play a trick during an emergency. Seat belt demo also serves a reminder to buckle up. Thankfully, never encountered (knock on wood) an emergency but have forgotten to buckle up once or twice.