Imho it makes sense for both Google on Android (Java) and Apple on iOS (Objective-C) to provide more (and more) modern alternatives to develop applications for their platforms.

There certainly are people who are happy with Java and Objective-C but there are many who'd like to use different languages/runtimes/ecosystems.

The popularity of projects like Cordova and CrossWalk show how much demand there is to build applications using web technology even if you have to pay for it in performance.

Sky is a way to use Dart without having to compromise on application performance nor on development feedback speed and debugging tools.

Also think of the people (whether large or small companies) who have to support multiple platforms (Web, Android, iOS, Server, …) simultaneously and right now are forced to use multiple languages and tools for building/debugging. This is incredibly costly, especially if you want to deliver high quality products.