Libations are simple: one pours directly onto the ground.
Food offerings, though, are a little more difficult. If there's a sacred fire present, one can burn them, but what if there isn't? It seems rude to lay them directly on the ground. (If I offered you a sandwich and set it on the floor in front of you, how would you feel?) To set out food offerings in non-bio-degradable containers pollutes both physically and spiritually. What to do?