NMC,
The answer to your question is well illustrated in the accident to Puma ZA934 on 8 August 2007 and the fatal injuries caused to a soldier at South Cerney some years before, when he was hit by a Hercules, whilst standing on the cab roof of a lorry. A less serious illustration is the Phantom flypast at Cranwell.
It's to see how far the boundaries can be pushed and what one can get away with. However, in these days when a simple mobile phone can record it all for posterity and send it round the world in an instant, it becomes more difficult to keep quiet. When it all goes wrong things are really serious.
Old Duffer