Don't quote me as I'm not 100% sure of all the facts, but I believe that earlier this year, air ambo paramedics were effectively stopped from administering some classes of drug, that previously they had been able to (my hazy recollection is some of the morphine-based painkillers).
So a properly trained doctor can for example intubate, perform RSI's and administer classes of drug that the paramedics are not trained for or cannot legally do.
It's not (just) a question of skills, but also legality. All of the air ambulance paramedics I have met are highly skilled individuals, but there is a glass ceiling of what they are allowed to do, imposed at national or local ambulance service level. And to a man (or woman) they do not regard the doctor as an inconvenience, they recognise that having a A&E trained registrar (in the case of the charity I know well) adds a whole new dimension to the service.
And Crab, I disagree with you. The point of an air ambulance is to get the patient appropriate treatment as quickly as possible. With the right team and equipment on board, this might be at the scene. Often the stabilised patient is then moved by ground ambulance - but effectively their treatment may have already begun. The doctor often travels with the patient in the land ambulance.