When an Aeroflot flight from Moscow to Bangkok hit air turbulence, 27 passengers were injured, some seriously. The flight was 40 minutes or so from landing, so there were bound to be casualties, with passengers waiting to use the lavatories and re-stowing hand baggage. Others were hurt just because they did not have their seat belts fastened.
On a plane you are like an egg in a metal bottle, jolted about if you are not secured. The plane has to only drop 100 feet, but it will feel like 1,000 feet and you will hit the ceiling and fall back onto the cabin floor.
Changes in air pressure, can cause a plane to bounce about in the sky. The wings will flex, the engines will look like they are about to break off, the fuselage will shudder and shake. The plane won’t break apart or collapse. You, on the other hand, are not built for this and will break open.
Keep your seat belt fastened, even when the seat belt sign is off.