Buying Dark Sky makes a lot of business sense for Apple. Apple clearly, since iOS 6 8 years ago, has been making moves towards bringing more of its default information sources in house, and buying Dark Sky for its existing infrastructure and expertise makes more sense than trying to start a weather division of their own. Plus, Dark Sky is digital native, so it means that Apple wouldn’t have to deal with legacy media partnerships like they would if they tried to buy Accuweather or Earth Networks (which owns WeatherBug). They were also probably paying quite a bit to The Weather Channel to cover all the weather queries on iOS, iPadOS, watchOS, macOS, tvOS, and Siri (and the HomePod). It looks like the Stocks app still pulls from Yahoo Finance, and Google is, of course, still the default search engine in Safari, but I think those and some of Siri’s content sources are the last few default services on iOS that aren’t owned by Apple. We’ll probably see Apple buying a similar company to power its Stocks app soon.
As a developer, I’ve used both Accuweather’s API and Dark Sky’s API. Dark Sky gave a lot of info, including the rain timing info, but Accuweather’s API was better for providing an extended forecast. Accuweather has a similar rain warning feature, though, so I’d expect Accuweather’s API to take over as the dominant weather API once they add features like rain timing to it.
