YouTube Music is adding lyrics support for offline playback. This makes it easier for users to view lyrics to songs without an active internet connection, and it's a feature that's yet to arrive on other music streaming platforms like Spotify and Apple Music.
Offline lyrics on YouTube Music will only work if users have some downloaded content saved on their phone. It will work for songs and playlists that a user manually downloaded, as well as for Smart Downloads that are saved by the app automatically.

As spotted by Android Authority, the feature has been a long-requested option in the community. Lyrics for songs and offline playback have been on the app for ages. So, it was only a matter of when YouTube Music would push the combination of the two features.
When users play content from Library > Downloads without an internet connection, they'll be able to access the offline lyrics from the same Lyrics tab at the bottom. The feature currently works on the latest version of YouTube Music on Android, but not on the iOS version yet.
With YouTube Music already rolling it out, it's now down to Apple Music and Spotify to release the same on their respective apps, since offline lyrics appears to be a frequently-requested feature on music streaming platforms.