During every flight, we always hear the cabin crew say "In preparation for take-off, please make sure that your seatbelts are fastened, window shades up, tray tables stowed, and seats in upright position" then followed by a "we will be dimming the cabin lights for take-off...". Same procedure also goes for landing. We may now be wondering why we have to follow all these, and the answer is simply because of safety.
Take-off and landing are usually the most dangerous parts in a flight and usually where most accidents happen. All those procedures our cabin crew instruct us to follow each have their own reason which boils down to your safety and the safety of other passengers should an unwanted emergency situation happen. Not following these instructions may cause major injuries or even lives if you do not obey them.
Never forget that it is the airline's and the flight crew's job to get you from point A to point B safely in one piece, hence, as passengers, we have to do our part by following these instructions. Safety is indeed paramount.