Like how everyone says, control, knowing that they can't do anything about their body being use in such a crude way by someone else, they'll fade into dust, others might live whit the curse of knowing but you don't have to live whit that, your victim is no different than a living -or well dead- sex doll, or that's at least how I see it, they really only serve one purpose and that is to give pleasure to one.
I find it hotter when the person having sex with the corpse is the same person who killed them. It's a bit of a personal thing since I know a lot of people prefer the fantasy of going into a morgue or centenary, but seeing their face of horror while their life spills away would be truly the best.
But like most people, these are just fantasies, nothing more, nothing less.