davep made an interesting post where he talks about the FSF reaching out to him and asking if he would be willing to hand over the copyright to his quickurl.el package so they could make it part of Emacs.

I was curious why the FSF asked needs to transfer copyright ownership when the GPL allows the user to do whatever they want with the code as long as they adhere to the terms and freedoms of the license.

So I emailed davep and he very kindly explained.

To better defend against license violations, the FSF requests copyright assignment for code included in their projects. Because US law allows only the copyright holder to enforce a license, the FSF assumes this role to ensure they can legally act against any breaches.

The FSF has a page specifically for this question, which davep pointed out. You can read it here.