There are definitely reasons for such a craft, but I think that unmanned ambulance is not one of the better applications.

If you are evacuating someone, you want have a doctor or paramedic on site to administer things like plasma, etc., and they would likely need to monitor the patient for the trip if it is any longer than about 10 minutes, and once you have to do that, having a pilot too makes sense.

That being said, things like remote resupply would be a viable application.