This dataset was created from the one collected here on HN. This was a semi-manual work, but we managed to make it somewhat uniform.
I think Indeed also collects voluntary salary data, but I don't know any other sources off the top of my head.
It's useful for people to be able to self-assess, I think. It helps round out your assessment of a) where you are in your career, b) where you are in your field, c) question if you're satisfied.
I'm not sure it's something that should be forced on people, but it's nice to know it's there if you go looking. Once somebody's looking for answers like that, then usually there is dissatisfaction already rooted... or just plain curiosity. Nothing wrong with curiosity.
But where it's largely against the rules, or a faux pas, to discuss salaries with coworkers, it can be hard to gauge your situation if it's a concern.