Just like with mobile apps, battery info can help websites decide whether they should run a complex operation now. It also helps detect regressions if a commit happens to suck power faster. Unfortunately, it's more bits available to deanonymize users across websites.
Data channels are useful for synchronizing your phone and your laptop. All native platforms support this. Most websites do it with a roundtrip to the server. So it would be mysterious to the user why a permission is prompted for something that should just work. Obviously it's bad for privacy though.
So painting it as having no legitimate uses is a bit one-sided. There is no easy fix.
What websites do CPU-intensive work that can be delayed for hours?
In general I want a website to suck as much power as possible so it can be done sucking power as soon as possible, whether I'm on battery or not. If you want to find waste, track execution time and bandwidth use.
I'm not sure I understand your point. Many apps (would) benefit from delaying some tasks and waiting to be on charge and a wifi connection. Typically, synchronization for example requires the device to be awake for a long period because of network roundtrips, transfers, disk seeks and CPU work. Software updates are similarly impactful on the battery.
At least, it makes sense to measure battery usage to optimize your code. And in my opinion, it's also beneficial to react to battery events (delay when on low battery, process when on a wall socket) and network events (bad coverage vs. strong wifi).
Data channels are useful for synchronizing your phone and your laptop. All native platforms support this. Most websites do it with a roundtrip to the server. So it would be mysterious to the user why a permission is prompted for something that should just work. Obviously it's bad for privacy though.
So painting it as having no legitimate uses is a bit one-sided. There is no easy fix.