In the early 1990s, a homemade VHS circulated around the underworld before there was the dark web, coined Grave Robbing for Morons.
In the eerie video a young man explains how to actually rob a grave. It stutteringly explaining how to rob the graves without getting caught, what bones are most valuable, and other grave robbing tips.
Showing what appears to be an actual human skull that he's stolen and at the end he gives the nicknames of himself and his grave robbing crew: "Anthony, "Gino, "Taco", and "Pucci" and vows to continue robbing graves for the fun and for profit.
To this day, no one knows who made this video or who the narrator is.
The value of human remains:
Grave robbing is a very old practice and needs to be divided into categories. Classically it's usually for the material items on the bodies not the bodies themselves.
In modern times it's been everything from wedding rings and teeth to the physical clothing they were wearing at death, in the pre-modern era it was for nebulous "grave goods" , swords, money, and other things buried with people tied to afterlife beliefs.
One who steals from physical bodies is often termed a resurrectionist and they do it for a number of reasons.
Reasons are obvious (desecrating graves of your enemies, concealing crime, etc), but financial reasons have varied considerably. Usually they are focused on one of two areas: selling the bodies themselves for science or selling parts of the bodies for religious ritual. As modern medicine and anatomy began to really take hold in the latter half of the last thousand years the primary method for doctors and students to study bodies was to buy real ones.
These were often provided by local agencies in an above board manner, dead inmates from prisons and the like. However official bodies were expensive and in demand and so they usually went to the most monied or skilled buyers and if you were a average doctor trying to bone up (harr harr harr) on skills you'd probably not be able to get one.
So persons in the medical field and scientist alike turn to resurrectionists, persons who stole or otherwise acquired dead bodies illegally for profit.
Famous serial killer HH Holmes actually dabbled in this as a method to dispose of bodies and famously once sold victims as a medical skeletons to colleges. Dead bodies were often used in many scams. Going back to HH Holmes, he would engage in what was at the time a semi-common racket: where you'd take out a life insurance policy on a person and then use a corpse you bought to fake that person being killed in an accident. These reasons for stealing bodies are obviously not really needed anymore.
We live in a world where it's much harder to become and student of any profession that handles the dead, and institutions have their hands on bodies that are ethically sourced and donated to science as well as having access to many complex and well made anatomical models and diagrams. So stealing bodies for medicine and scams is now, mostly, obsolete.
As for religion? That's where things get a little more muddy. Most religions, have a history with the body parts of dead people.
The Catholics have their reliquaries, the Buddhists have their mummies and other preserved things, basically only the Zoroastrians are the ones who have been around a long time and are also historically anal about NOT carving off chunks of people (several other religions are like this too). In the modern era however we're basically down to just the black magic, witchcraft, pagan dark arts and voodoo in kind.
Usually the darkest and most powerful spells/rituals are the ones involving human corpses.
The combination of rituals involving human remains often crops up in syncretic religions where you get things like Voodoo and Eastern European witchcraft combine ancient old world religion with more modernized Christian or Catholic beliefs. Many tribal non-monotheistic religions have a very different view of power, seeing the world not as a duality of good versus evil but instead as a more gray spectrum defined by choices. In many of these beliefs there exists the choice to trade ultimate power for morality, and often these rituals involve human remains.