If you were of a mischievous frame of mind, you could have a lot of fun with this bit of protocol. It's of vital importance to the Royal family's image and standing that they appear gracious and accommodating at all times. Therefore, if you present them with a gift, they're obliged to accept it and thank you for it. They may dispose of it as soon as they get home, but right there and then they can't say no!
This has led to them accepting some truly bizarre gifts over the years, including an Arctic Monkeys album, a miniature Iron Throne (from 'Game of Thrones'), Hilary Clinton's autobiography, a German canary and a full-sized Cameroonian elephant, which was ultimately cared for by London Zoo. It cannot be confirmed or denied whether Clinton gave the Queen the autobiography herself.