I don't know the answer to this either. I find it annoying because the sim is dead so they don't need to eat, use the bathroom, etc. I just do it so I don't have to listen to them complain during their duration before being sucked into the underworld. I looked online for an answer but couldn't seem to find one. In previous games (the ones I've played) ghost sims didn't need to eat. But, I was able to use my cat to capture one of my sim's family members so I had their tombstone, which I could place in the yard. Like the cemetaries, the ghost would come out at night around 9pm or later and it would be uncontrollable. That was nice. But, sometimes they would stay out past dawn and be there all day so having my sim's cat threaten to capture the ghost usually sends them back to their grave. (lol). Most of the time I usually send my dead sim back to the underworld so I don't have to deal with them anymore and can move on.
