When most of a society's dead are cremated or buried in ordinary graves, bodies placed somewhere else instead point to a different kind of death. Corpses left in bogs, many strangled, hanged or with their throats cut, stand apart from normal funerals. That contrast is the core evidence that they were deliberate killings, read as sacrifices to a fertility goddess.
Under the right conditions a corpse keeps the ordinary details of a life: stubble on the chin, the rope used to kill him, the barley gruel he ate last. Read carefully, these traces let one reconstruct how people lived as well as how they died, including their clothes, food and rituals.
Finds made before proper excavation and preservation methods existed are mostly lost to understanding, even when the objects survive. Of more than a hundred and fifty bog bodies, little can be learned about most because they were found in the 1800s and early 1900s. Recovering a find well, with careful conservation, decides how much can ever be known from it.