As described in the answer for a question similar to yours:
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.
And, as Apple describes it, you can use UTI's to describe an entire class of entities with a type (such as the format for files).