Hi Andrew. Yes but no
Imagine I'm recording a learning courseware. I split a specific topic in 3 videos. And I set Unity Lightmaps ( Part I ). Unity Lightmaps ( Part II ). Unity Lightmaps ( Part III ). In all of those videos the most important tags are Unity, and Lightmaps, but also the part of the video it is true that it is important, but probably for the guy who is looking for something like unity lightmaps, he's not aware about my video is split in 3 parts. So it is important to be able to add tags or similar tags to videos. In my case I review my tags, as you can guess, from a SEO point of view. You have to take a look to your competitors when we're talking about SEO. That means, you can decide to add some tags in specific moments. So for many videos in the same topic, you can consider it could be interesting to add several tags, that even they're related to your content, you didn't add in the moment of video uploading. Most in my case where most my videos are live always. So I hope you understand that the tool to add tags, is quite good to ADD, not just to replace all the existing tags. So add tags means, you can create a group of tags you consider related to many of your videos. Even taking in mind that the most important tags already added are the one that make the difference.
I hope you understand me. I'm not native english.
Thank you very much.