When developing for iOS, watchOS or macOS you can provide 1x, 2x and 3x versions of an image using PDFs. But you can't specify your app's icon in the same way. Instead you have to manually create a number of icons for spotlight, notifications, settings, the app itself and, in Xcode 9 the iTunesArtwork, all at different resolutions.
It's time consuming, and frustrating. With APIconizer all you need is the PDF of your app's icon. Drop it in and then APIconizer will do the rest for you.