seems like the screenshot of the page isn't support in most of the website
I think that when the user bookmarks the webpage, the webpage is cut into a certain size screenshot on the browser side, and the middle can be intercepted or stretched appropriately. Then sent it into the server.
This can reduce server stress and allow forgetful users to find sites that are hard to find.
And the most important: Many users (Likes me) prefer bookmarks in the web view instead of images, which are easy to find and intuitive.