I don't feel like this is of any use
someone could scrape their knee and it would be the worst pain they've ever felt, and they would say ten. but it's only a scraped knee
That's statistically unlikely.
And scraping a knee might be the most pain you've personally ever experienced, but it's easy to contemplate more pain, or pain great enough to debilitate your further (which is more akin to what you're asking for with such a question).
And if you can't imagine being debilitated further then you're likely at a 10, and scraping your knee is pretty unlikely to leave you in a situation where you feel that way.
Either way it's a better question than none, and since pain is subjective and also not even necessarily based on injury, it's the best way to scope it. All medical professionals use such pain-scales because there is nothing else.