I've never used the voting. It's a quality signal and they are notoriously difficult to get right. Netflix infamously switched from stars to like/dislike and everyone hated it, and yet it provides better signal and recommendations become better and therefore you get better content surfaced that you'll enjoy more. I'd guess a similar thing happened with YouTube.

Zooming out there's typically an ecosystem benefit, because otherwise why would they do it. That ecosystem benefit isn't always about money, if no one watched videos there would be no business. In fact a business that is often much better than a business with no users that isn't, so user happiness and experience tends to be prioritised highly in products/companies like this.