One reason is the complex relationship between Snape and Harry in the original Harry Potter series. Writers are intrigued by this relationship and use the idea of Snape pinning something on Harry as a way to explore it further. It could be a symbol of protection, a sign of a hidden connection, or a device for plot development.
I'm not sure exactly what the specific story is about hair pins and watches. It could be a story where a hair pin is used to fix a broken watch strap, or perhaps a more magical story where a hair pin has some special power related to a watch, like opening a secret compartment in it.
Queen Pins is not based on any actual events or people. It's the imagination of the writers and creators, crafted to provide an engaging and fictional narrative for viewers.