Every once in a while some random account forks one of my repos, and when I look at the account to see if they do anything interesting I see dozens of empty forks, many of which seem to be created within a timespan of a few hours or so.
My guess is that some people use them as bookmarks... but isn't that what stars are for?
Making a fork also lets you keep a copy of a repository if the original public GitHub repo is deleted or made private. So in that sense it's more useful than just starring a repo.
My biggest annoyance with github is that everytime you accidentally click the edit button on a file it instantly creates a permanent fork under your user. I must have dozens of them that i never bothered to clean up