Most likely for generating unique fingerprint for tracking

Maybe it uses some that battery API as a heuristic for a lower-power version of the site? Or maybe they have a web-only version in developing markets? Low battery means it should query for your location less often to save battery?

Totally spitballing here. Strava being a website that requests battery does not seem wildly outlandish to me, albeit it is a bit suspicious in general.

Strava's a route tracker. Assuming you can use it through the website, it probably controls how often it polls location, trading off accuracy for power consumption.

Bots trying to brute force accounts may not have the API implemented like a real device may.