IP addresses are important because they tell someone exactly which physical computer posted the message.
If there was ever a serious problem, the admin could trace the address to see where it came from. Once the ISP was determined, they could, if necessary, find out who sent the message and turn the information over to the proper authorities.
Other boards publish the IP addresses for EVERYONE to see.
i'm not sure of the significance of hiding them from the masses. It would help weed out the imposters, spammers and jerks who come on the board.