[William]

First, he might simply be studying them to advance his medical and magical knowledge, much as real doctors did with cadavers (stolen or legally acquired). A large library of organs from many different races, ages of people, genders, etc. could be a useful reference.

Are there any magical beasties running about with useful organs? Eye of newt, heart of manticore, and all that. Come to think of it, the whole organlegging might have some history if there's any way that chimerizing oneself with exotic organs is possible and conveys some benefit.

Flesh golems are mentioned in passing under the description of the Golem spell in Magic. Your necromancer might have had an interest in constructing these as servants a bit sturdier than zombies. You'd certainly need a steady supply of organs, preferably as fresh as possible.