It's a position, just as any other position is. Their role is very different though as the scrum is marginal in the general running of the game. The main role of the prop (in attack) is to get his team up the field. They tend to be very big with a decent turn of speed, with George Burgess a prime example of such a beast. Some also have a decent handling game, with James Graham an example of such a prop. Andy Farrell was also playing as a prop in his final season in league.
I'm in two minds about Sam really. The man is an undoubted superstar, and is an incredible rugby player. I'm just not sure how his skills can be utilised really. His game is based around action and physicality. He's always running the ball in hard, looking to punch through the line and offload, or he's looking to hurt the opposition with his defence.
Those things aren't really a factor in union. Compare Sam to someone like Brad Barritt, who will be a rival for a centre spot for instance.
Barritt makes around 10 carries per game for maybe 40 metres gained. Sam averages 16 carries this season and 144 metres gained. Likewise, he's averaging 30 tackles a game, which no player in the England team gets close to, and certainly not anyone in the backs.
Those are his strengths. He doesn't have any kind of kicking game. He isn't a playmaker. SBW did ok for the All Blacks because they play a style of rugby that is probably closest to league and regularly had support runners to get on the end of his offloads. England don't really, and still revert to 10 man rugby when the chips are down.
For what it's worth, I watch a fair bit of rugby, and for me, the NRL is the best competition and standard of rugby anywhere in the world. The intensity, the brutality, the skill, the flair. Unmatched.