When developing an app or a library, it’s pretty common that at least once in the course of development, you’ll need to conditionalize compilation of your code. Maybe you’ll be accounting for a bug in the operating system where things that don’t work quite the same on your device as they do on the simulator. Or perhaps you’ll want to simply exclude code from your simulator builds because the simulator simply doesn’t have that functionality (like invoking the camera).
— Read on davedelong.com/blog/2018/07/25/conditional-compilation-in-swift-part-1/