[deadbart]

I think beetman really hit the nail on the head with this one. Another point is that depending on the exact type of game, raising preflop may make it much more likely that you get paid off on your nut hands. Some games are loose preflop but not so loose postflop, so raising preflop really helps build bigger pots with hands that like it, and encourages people to stay with second nut lows and such.