Royal Family Protocols

The Queen uses her purse for a myriad of different things, most of which is to signal to people when she's done being in their presence. When Her Majesty wants the room to know she is about to leave the dinner reception in five minutes — signaling the end of the event — she will put her purse on the table, allowing people to finish up their desserts, stat. 

On different occasions, she will move her purse from hand to hand to signal to her assistants that she no longer wants to be in the conversation that she is in, alerting them to pull her out of the chat.