The 1080p Premium tier offers a higher bitrate than the standard 1080p option, which means that videos will play with a higher quality and less buffering. This is especially beneficial for users with high-speed internet connections and large TVs.

While all users will still have access to 1080p, this enhanced 1080p quality setting will look extra crisp and clear, especially for videos with lots of detail and motion.
– via blog.youtube
The introduction of the 1080p Premium tier, and the fact that it is now being expanded beyond iOS, is a sign that YouTube is committed to providing its users with the best possible viewing experience. It remains to be seen whether the new tier will be successful, but it is a welcome addition to the YouTube lineup.