I would say it's to keep people in their seats and give a clear signal when they can get up.

I can't count the numerous times we've had to stop short of a gate and wait for somebody to switch on whatever it is they have to switch on before coming to a final stop. Needless to say, even when the captain has warned people, I've seen people get up. Aircraft brakes are powerful and even with what I would have thought is a 'gentle' application, you see the silly idiots fall over!

I have to say that the BA cabin crew are generally excellent at telling people to sit down. They have my full support and I don't relish a litre of gin smashing on my head when some fool opens the overhead bins too early! Unfortunately I've had the worst experiences with IB who don't seem to care at all when people get up too early.