It's unclear precisely what Apple plans to do with Dark Sky, but there are plenty of possibilities. Apple already has premium apps — either developed in-house or through acquisitions — for many popular app categories. These include music (Apple Music), news (Apple News Plus), streaming entertainment (Apple TV Plus), personal finance (Apple Card), and gaming (Apple Arcade).
Adding weather to the portfolio seems like a sensible addition, especially considering Apple is rumored to be working on a bundled offering that would include multiple services for the one price. A subscription that provides a premium weather app along with the news, for instance, seems like a natural pairing.
There's also the possibility that Apple could use data from Dark Sky simply to bolster its existing core iPhone apps, like the weather app and Siri. Apple's stock apps have been the focus of its major iPhone updates in recent years, so it's plausible to think that Apple could use Dark Sky as part of planned improvements to the weather app in iOS 14 and beyond. Last year's iOS 13 update brought major upgrades to other Apple apps like Health, Reminders, and Photos.