We must come to understand that the energy we need to survive exists all around us. However at this point in our evolutionary process we can not simply absorb the energy, though certain practitioners of Buddhism and other such traditions have (through certain techniques) achieved this feat. It tends to happen during extended periods of meditation. However… these people to a large extent still rely on food and water.
So in essence the food we eat and the water we drink are simply vehicles for the energy we need to survive. From our perspective this rings true. Therefore that is exactly what these things are due to our presuppositions when used as offerings to the Gods. They are simply vehicles of energy that the spirit can rely on for manifestation. We think we are feeding it so we do, and therefore the spirit can fuel our desire in return. It is our intent and subconscious “knowing” of this fact that allows them to work. The offerings are usually specific to the spirit being worked with to create a more specific gateway of resonance. I know from experience that any offering will work if the correct intent and strength thereof is funneled into the offerings. In fact to begin with using such simple things as bread and water will help train the adept to funnel such energy and intent. In this way by the time one begins studying which offerings are specific to the spirits the results can be much more intense and refined.
This is why simply putting offerings out may not suffice. Consecration of the offerings is a must because the aethyric double of the offering must be assigned a purpose just as any other instrument of magick and/or sorcery. This is the practical reason for consecration in the first place. You are filling a lifeless blade with intent and purpose which brings it to life for example, when consecrating the athame. The more you use the tool, the more the aethyric double of that object is charged. It becomes a battery which accumulates energy which can then supplement the energy raised within the magickal rite itself.
Offerings in the same way reciprocate energy between you and the spirit summoned. However I feel it should be assigned to do so. Their is a common denominator pointed out here which is often overlooked. When you see it though… it is easier to see the practical reason for these things. Then it is easier to see that offerings made unto the spirit are also offerings unto ones self due to the reciprocal nature of the energy. Offerings like all magickal tools are a means to further refine your intent or act as gateways to weave the aethyr according to desire.