Why do we offer prasadam to Gods, fully knowing that they don't eat it?

The life is a spiritual journey carrying cosmic energy (God) through the material world using body as the vehicle. So the God is actually inside and not outside. The God within us is the same as Cosmic force that we worship in various shapes, forms. It branches out from there, and goes back there after completing the earthly journey. That is advaita. 
If we have God within, then why do pooja? Like the body that needs washing daily to rid itself of the sweat and dust, the God within needs to be cleaned of the earthly wants and desires that get stuck to it. While doing pooja, we imagine the God within us being installed in a outside form, wash (abhishekam) and after cleaning, dressing up; reinstall within us. 
So, that God is who needs Prasadam!