When you're traveling you probably like to throw a swimsuit in your bag just in case you get the opportunity to go for a dip. However, the royal family must always pack something else in their bags ... and the reason isn't quite so fun as hot tubbing.
If you watch "The Crown" on Netflix, you probably remember when Elizabeth has to change into black mourning clothing before exiting the plane when she is informed of her father's death during her Kenyan tour in 1952.
On this momentous occasion, Elizabeth did not have any black clothing with her so she had to wait until someone brought it before she could leave. Otherwise, she would have been inappropriately dressed on that occasion.
According to The Independent, since then, it had been mandatory for royals to bring a black outfit whenever they travel in the event that someone should die and they need to change into proper mourning clothes.
It's something to keep in mind when you're deciding if you can squeeze a 6th pair of shoes in your bag for your weekend trip — at least you don't have to plan on packing an entirely black outfit in case a loved one dies.