The reason is marketing. If your website sells you as the perfect fit for everything from embedded systems to web apps to iOS to Android, you are going to have a hard time to stick out in any pile of candidates-for-our-next-project. If however you wrote a book on iOS development, have a few dozen in-depth articles on Objective-C and Swift on your blog and can refer to a handful of your apps in the app store, you will at least be on top of the (lucrative) pile of candidates-for-someones-next-iOS-project.