There are many things that are forbidden to board a plane and these things are well known to us humans. So you have to wonder why there are people that are actually trying to bring forbidden things with them inside a plane. This is actually a trivial concern for aviation companies despite having numerous reminders.
You try bringing one of the featured things and the delay of your flight will be rest assured. It seems that the moral of this is to remind people that no matter how good they are at hiding the things they want to bring inside an aerial vehicle like planes, if these things are forbidden, authorities are bound to find about them.