When posting a video on YouTube they ask the user to enter tags. I think the purpose of a tag is to help search engines like google discover a web page. However, the tags on YouTube pages are visible for visitors to read. Do they have to make them visible for humans to see? Why would they do this? Thanks.
Well at times visitors also click on those tags to find videos of similar genre/interest. Like in the above pic I want to click on BMX to see more biking related videos. So yes visitors also use tags apart from what you said, that is being used by search engines.