YouTube has been testing this feature since May last year, but the latest update marks the first time what they call 'original sharing' to everyone.
The main purpose of this feature is to make YouTube more social, encouraging users to chat about videos right inside the app instead of other apps (like Twitter). This brings obvious benefits to YouTube: if received, it will make people spend more time on YouTube.
And perhaps better yet, it gives users a private space to comment on videos instead of "flooding" in normal comments.