when you want to watch a video tagged as 18+ only (because for that you need to log in).
You don't need to log in if the video is embedded on a page, or if you open the player directly like this (/v/<id> in URL): https://www.youtube.com/v/UF8uR6Z6KLc
You don't need to log in if the video is embedded on a page, or if you open the player directly like this (/v/<id> in URL): https://www.youtube.com/v/UF8uR6Z6KLc