You could ask my aunt, since she is alive, about it.
Her car flipped upside down into a water-filled ditch, breaking the windows, and instantly filling the car with water.
If she had had to fumble with unfastening the seatbelt, chances are overwhelmingly slender that she would have made it out.
Of course, everyone has an relative who would have been dead if he had been wearing a seatbelt. It's always the case.
I, on the other hand, have been in 3 accidents where there has been a person killed. On my count, there were a total of 11 people involved in those accidents. 4 dead, 1 hospitalized, 6 walk aways. All 4 of the dead were not wearing seatbelts. All 7 of the survivors were.
So I'll take my personal experiences, weigh them against your aunt's miraculous survival story and come to the conclusion... wearing a seatbelt increases your survival odds by a gigantic proportion.