Think how Google/YouTube understands what is in your video. The bot that determines where you should place in search results can't physically watch your video. It can't look at your video and say "Right, this video is a review for the Playstation 4", because it can't watch your video, it's a robot. So, your tags, title and description tell the robot more information about your video. As much information as possible to help the robot read and understand the contents of your video.
Tags help make sure that your video that is about a Playstation 4 doesn't come up in the results for "cream cheese", for example.