I opened Xcode's Organizer, clicked on the documentation, and typed in "Mobile Core Framework" in the search bar. The first result is this:
As I recall, the times you need to use UTIs is if you're sharing files between applications. You'll need to use UTIs to open files downloaded from Safari or Mail or transferred to the device through iTunes. You'll also need UTIs if you'd like to send data to another app, like Mail or to transferred to a computer through iTunes.