You might think that a purse is just a purse, but when it comes to royalty, it is not. The Queen uses the bag to signal to the staff that she is upset by the conversation. Princess Diana used it to hide the décolletage in order to avoid paparazzi photos. And if you look closely at Kate's photos, as well as Meghan's, you'll notice that they hold the bag in their left hand when present at formal events.
This tactic is part of the royal protocol, as the right hand must be free so that I can seem to meet or greet people. Whereas, when kings do not want to give a hand to someone, they carry the bag with both hands. So from now on you can easily tell if Kate wants to greet someone.