I guess they're useful when the content is of low quality but you're desperate to keep eyes on the name of your site.
I hate them.
About the only thing worse than self-hiding vanity flap is a floating side bar using a slow animation to slide back into your field of view, just terrible.
I have a special section in my uBlock user rules, and it is full of annoyances like this.
Another one is an element with popup on hover, but you can only close it by hover-unhover a popup itself. Fly across such element by accident and enjoy your semi-modal “no idea how to get rid of”-popup without even clicking anywhere.
So many hours spent on nothing when a simple `viewport-offset-{top,…}: 20px` could solve it once and for all. Instead we will support global nonsense described in the article for years, despite it stays nonsense at any angle.