Prototyping mobile accessories! When you're working with hardware, building things to make things talk takes a lot of time. This makes it easier.

It's also useful for student projects. I got one of the original wiretaps when they were giving away their first batch for a prototype medical device (we didn't end up using it in the end because it was still too much work).

Well there is a small company that created a credit card reader using it to enable smartphones to act as Point of Sale machines. They call it "Square".