To be able to load a less resource intensive version (i.e. disable animations, reduce refresh rate, remove canvas elements etc...) of a website if the battery is low.
> “Some companies may be analysing the possibility of monetising the access to battery levels,” he writes. “When battery is running low, people might be prone to some – otherwise different – decisions. In such circumstances, users will agree to pay more for a service.”
That describes a scarily large portion of all web stuff.
This lets sites who want to do "real things" with battery levels have access, without it being available "for free" (prompt is annoying).
My guess is this is just another part of the attempt to move desktop features to the web browsers so that people don't have to write actual applications. Personally, I don't like this trend.
For the ideologically inclined, it’s also about disintermediating the relationship between users and developers. Web sites don’t need an app store.