why were peter, isaak, rick, and jake envied by the other slaves?

"My master's son-in-law, Captain Auld, was master of the vessel; she was otherwise manned by the colonel's own slaves. Their names were Peter, Isaac, Rich, and Jake. These were esteemed very highly by the other slaves, and looked upon as the privileged ones of the plantation; for it was no small affair, in the eyes of the slaves, to be allowed to see Baltimore."