Before the abolition of monarchies, Zeus was the protector of the kings. Once greek kings faded in democracy, Zeus became chief of judge and peacemaker, but most importantly civic god.
There was a couple of ways to celebrate Zeus when he would do something heroic like throw a party and give up things to give him. By doing that the would make a fire and what they were giving up they would throw in the fire, the people believe that if they throw something in the fire it would go to Zeus or any other god they were celebrating.