It's really what the coroner's assistant is called.
To quote Wikipedia:
The word Diener is German for servant.[1][2] In English, it is used to describe the person, in the morgue, responsible for handling, moving, and cleaning the corpse. It is derived from the German word Leichendiener, which literally means corpse servant.
Dieners are also referred to as morgue attendants.
They do all the grunt work.
As for demons in the Buffyverse...they were treated as metaphors for life's real problems. It's what most science fiction, horror, and fantasy really is anyway: an extended metaphor for real life.
