Turbulence can hit unexpectedly and when it does, it can be injurious or even deadly. A recent Avianca flight from Lima to Bueons Aires encountered unexpected but severe turbulence at 41,000 feet, throwing passengers and crew across the cabin and resulting in severe injury to a pair of FAs.
Take a look at these two pictures–
Photos / TrafficAirColom via Twitter
The point is, even while turbulence can often be anticipated, sometimes it cannot be and when it occurs you can be severely injured when not buckled up. Having encountered severe turbulence before (though not this severe), I make it a policy to remain buckled while seated, even when sleeping.
Not tightly fastened, but just sufficiently buckled so that if the aircraft begins to violently shake I will not find myself banging my head against the ceiling.
Just like it is common-sense to place your seat belt on when in a car, it is foolish not to take easy precautions when flying to protect yourself.
More photos here.