If your channel is set to "mature", then all your VODs and Clips are also marked as requiring a "mature" confirmation to view.
However, a clip from a "mature" stream may, independently, lack any mature content. And it could be that an otherwise family-friendly stream had a somewhat mature moment that was clipped, and rather than removing it to keep the "not-mature" rating on the channel, being able to mark the independent clip as mature would be much more functional.
Additionally, a streamer may have a "daytime" and "night-time" stream schedule, in which case one stream and its associated VOD may be family friendly, and another may not. Being able to mark the VOD one way or another would help make the "mature prompt" feature better at protecting those it's designed to protect, while still letting them enjoy Twitch and its streamers.