Few things are more dispiriting than turning the ignition key and hearing nothing. Just when you need your car to start, it plays dead. Worse still, it's probably your fault. Look at the lights switch - did you leave them on?
Modern car batteries are very reliable, but leaving the lights on for a few hours will soon discharge them. Even a boot light or an interior light left on overnight can flatten a battery. And a car left standing for a month or so may discharge its battery just from the current drawn by the alarm.