Most creators upload at 1080p, and YouTube has always given bitrate recommendations to match their service. Even with updated documentation, I'm expecting lots of people using their existing encoding presets for future 1080p uploads that can't benefit from this paywalled upgrade.
It's not better than the 1440p or 4K downscaled. The main use I see for it is for videos that were only uploaded in 1080p in the first place, and so they don't have a 1440p or 4K option.
But even then, I am not seeing a very big difference at all between the regular 1080p and enhanced 1080p.
1. If you are bandwidth limited the high bitrate version should be a bit more efficient than the higher resolution (as it isn't wasting space on details that will be lost in the downscaling).
2. For videos that are uploaded in 1080p it will be the best quality available.
So it does seem like a pretty niche benefit. Maybe if you have a 1080p phone it can be worth it for the reduced resource usage and better battery life? But with many channels starting to upload in 4k it does seem like a very minor improvement in most cases.