YouTube Music Key is not just for YouTube's mobile apps. One of its feature is also available if you use YouTube's desktop site: ad-free music videos. You'll notice the "ad-free" label next to the video title. Mouse over the label and you'll see this message: "Your YouTube Music Key subscription lets you play this video without ads."
"Subscribe to YouTube Music Key and you'll be able to listen to music without seeing or hearing ads. Ads won’t be shown before or during eligible music videos you watch on youtube.com, and music videos and playlists will play continuously on your mobile device, without interruption. You will still see ads on other YouTube videos, however. The Ad-free badge tells you that a video will be ad-free," informs YouTube.