To stop people from using apps they haven't paid for. As an honest person, if you want to use an app, you'd pay for it. Unfortunately, not everyone out there is honest, and there are various ways to get around having to pay for an app that costs money. Fingerprinting the device lets sellers of software find people who didn't pay for the software but are somehow using it.
I really hope you're being facetious. It' be pretty clever if you were, but for those that think this is serious...
If you want someone to pay for an app, don't make it free with in-app purchases. This is not something allowing for the OS to provide a unique identifier that can be abused available to app developers. App developers cannot be trusted. At. All. Ever.
