Nah, I think they do it for the devilment - annoying other road users whilst trying to work out how to murder the next prostitute
Actually I come from a family of hauliers (I'm not a murderer!!) so have lots of sympathy for long distance drivers. It may be that 10 miles back the poor fella was stuck at 40mph behind the lorry in front on a hill, and is now seizing the slim window of opportunity to overtake.
I drove a van during uni holidays and it wasn't much fun. Imagine watching the rest of the world driving around carefree while are limited to a certain speed and needing to be at pick-ups / drop-offs in time with all the problems of traffic on our roads - it wasn't for me. An empty flat-bed Mercedes 310D Sprinter does give a good insight into rear wheel driving though
The biggest problem as already mentioned are the twunts who insist on sitting in the middle or outside lane with nothing on the lane inside. I usually make a point of returning swiftly the the inside lane after overtaking them. Occasionally they get the point and pull in - more often than not they just sit there, clueless or pig-headed