Not quite that cut and dry. Take an evocation of Belial for example according to the Goetia an offering and a sacrifice must be made to him, so in that instance it may be something Belial needs or wants in order to act or comply with the Mage. This was a demand from the spirit possibly as a sign of obeidience and turning yourself over to him or he may need the energy offered.
Some Baneful works that call for animal parts may just be symbolic like hair of a cat and dog to signify fighting. Others have a harmful quality to themselves, something like goofer dust is made of dried scorpions, venemous spiders and snake sheds and snake heads mixed with graveyard dirt and sulphur. All these things were deadly in life so in magickal logic they’ll be deadly in spirit, the graveyard dirt was especially bad back in the old days before caskets were sealed and a slabs placed over them. The bacteria and diseases from the decaying body could be found in the soil for years after death. Which was another reason spiritual bathing was so important, you needed to get that shit off you and pray to god nothing infected you lol. You would put stuff like this out we’re people would walk through it and they could actually get the infection through their feet especially when people walked barefoot in the old days.