I used to work for a company whose primary product was a web server that companies could buy and run for use purely internally. Our pages involved a lot of data entry that could be lost, so that sort of pop-up can be handy in that situation as well.

Of course a better solution wouldve been a program which doesnt so easily let you lose data in the first place, but this software was long past that.

Still, that functionality should be opt-in.

It's really not that bad. The big problem was historically the buttons were labeled "Cancel" or "Ok", and some browsers allowed pages to customize the button labels, making which you clicked very ambiguous. Browsers today just giving "Stay on this page" and "Leave this page" buttons aren't really much of a bother if you remember how bad it used to be.

Or potentially losing any page state when navigating away or closing a browser, such as form data.

Right. You don't want to lose the blog post or other document you've been working on for an hour because you hit the close button on the wrong tab.

Websites can find better solutions if we kill this annoying feature. We have local storage in all browsers. Rather than prompting why not save it and recover when the user comes back. The users will always prefer this. It saves data even when the website crashes or the connectivity is lost and there is some important data on the page. Why have a feature that is abused more often than used especially there is no case where it is the only/best solution.

It's not always that clear. If I leave a page, how is one to know whether that was intentional or not?

If it's intentional, you don't want to pull the data back up (people want a "fresh copy")

If it's not intentional, you do want to pull the data back up.

Though of course you can make something like a "New Copy" button, but then that presents its own challenges.

That no longer makes sense unless it isn't feasible to preserve the change history along with current state...

I agree. This thought process comes from a different era and has just continued to perpetuate itself. If you are working on insanely* large files then that box makes sense, otherwise we have the technology to save the user the headache.

*insanely large is anything that takes up 75% of whatever computer bottlenecks first at the time of reading this comment(Memory, processor)