Another extract from my recent correspondence with Charlie of Imaginary Hallways, this time covering the reasons why projectors (people traveling from the living worlds) come to Necrocarcerus.
"1) Necrocarcerus sucks in people from lots of different eras and worlds to become Citizens, and it incarnates them more or less randomly, rather than in sequence. That is, Necrocarcerus isn't running at a parallel time stream to the living worlds, such that on say, Year 1 everyone who died in 500 AD becomes available for incarnation, and in Year 2 everyone from 501 AD becomes available. Rather, everyone gets jumbled together. So imagine you could travel to it and not only meet your ancient ancestors, but also your descendants (and even your own dead self). Or you could find out how an important war was going to go by finding an ancient sage who hasn't even been born yet but who will one day be the world's greatest expert on it.
2) Similarly, Necrocarcerus is in kind of a post-apocalyptic shambles, but it's still got fancier technology and magic that most of the living worlds do. The living worlds are all at different stages of technological development, and followed different cultural and technical pathways throughout their existence, so there are all sorts of innovations and advantages to be gained by sending people in to learn about or steal stuff. Your kingdom may be a podunk feudal backwater that's just discovered steam power, but you can send someone to Necrocarcerus and steal a nuke or a laser gun or a superpowerful death spell or whatever.
3) There are beings in Necrocarcerus who are interfering with the living worlds. The biggest one in my games is an undead necromancer named Thazul, who lives in Necrocarcerus but has created undead hordes that are busy invading three different living worlds (where he's their Sauron-equivalent). So brave paladins and champions are projecting in from the living worlds to try to slay him and save their worlds. There are other, less extreme examples, but from the perspective of many of the living worlds, Necrocarcerus is a hellscape full of undead who deserve to be smote.
4) Rare and weird items. Beyond technology and magic, Necrocarcerus produces a lot of unique materials that don't exist in the living worlds. So some projectors come in to conduct trade for things like soul coal, radioactive dragon bone, perfected spider eggs, etc. with unscrupulous merchants. This is also sort of the answer to the blood question - projectors are trading lots and lots of blood for stuff they want.
5) Religion. There's at least one god from the living worlds who's imprisoned in Necrocarcerus (Vra-Krakorn, He Who Consumes the Works of Mankind), but there's also a ton of souls to be won to faiths. I imagine a lot of these projectors think of Necrocarcerus as something like Purgatory, and are trying to offer the doomed inhabitants one last chance to get into Heaven / Nirvana / Paradise, etc.
6) Exploration. Imagine the afterlife was a real place, after all. You'd want to know everything you could about it - whether for religious or economic reasons, or just plain curiosity. A lot of projectors are just exploring the land of the dead to find out what happens to people after they die.
7) Guarding things. Necrocarcerus is a good place to dump that unspeakable ancient evil contained in that artifact sword that you can't destroy. Or you might need to guard the portal to the afterlife that you can't close from potential hordes of undead pouring through unexpectedly. One of the groups the PCs have encountered in my own campaign are the Knights of Tollen, who are an ancient order of paladins guarding a portal that leads from a backwater part of Necrocarcerus into their world.
8) Rescuing / resurrecting people. After all, if you can find your lost love and bring them back to the living worlds, you can be with them again. Or you can bring back your beloved but dead king to restore justice and depose the evil vizier who seized power after murdering him."