On some calls, a patient requires transport to a hospital that is a very long distance away, and because you don't know what a patient's needs prior to the call and you may only have a ground based unit on scene.
So as opposed to simple having that ground based unit drive and take an extremely long time to get there and then all the way back, it would be nice to be able to send a helicopter to the call after the patient had been treated and have them transport them instead.
There are also times where I send both a ground unit and a helicopter to a call and I can't specify which unit will transport the patient.