In past practices of necromancy the practitioner felt it necessary to use the bodies of the dead to force the spirit to return, and communicate with those that summoned it from it's realm. Among some of the haunts on a necromancer where cross roads, battlefields, and graveyards.
C
ross roads where selected due to there alleged link with the spirit realm. Often criminals where hung at cross roads in Europe. Generally though the problem with the cross roads is the lack of a corpse this would mean the body of the dead would have to be brought from it's resting place to where the ritual's where to be preformed. Battlefields provided a superb location to find corpses with one small set back, where there was a battlefield there was a war. It was one thing wanting to speak to the dead, but it was quite another risking the chance of becoming deceased by a hostile act of war. So by far the best and safest place to acquire a dead body was from one of the many surrounding graveyards. . . .
Not just any graveyard would be suitable for the needs of the necromancer, as being able to hide ones acts was most important. For this reason sites with crypt's where generally sought out. Even if the body of the person who deceased was not buried in a crypt it would take little effort to dig up the remains, and transport them to a suitable crypt during the hours of darkness.
There are other reasons besides remaining undetected for choosing a crypt. Another concern was that of shelter. The crypts not only provided a door to keep the rituals private from unforeseen guests, but would also provide a home for the necromancer, and his assistant during the lengthy art of raising the dead.
For some practitioners of necromancy the graveyard was only considered to be a shopping centre where they could pick and choose the cadavers, and there tools of the trade. These persons for what ever reason didn't practice there art where the bodies where buried. Whether they didn't feel secure in a public place, or they felt the need to separate the body from its resting place.
In modern times anyone caught desecrating a gravesite in such a manner would be arrested, andheld accountable for there actions in a court of law. Where a jail sentence would be handed down by the judge. In the Middle ages anyone caught doing these acts might be decapitated. Perhaps that's one law that should of not been changed.