need2no this is my theory.
If we believe in a god that takes care of us and blessses us with good things then it follows that we will want to give something in return, it's part of our instinct of reciprocity(We exchange gifts with each other). Think of it as an exchange of gifts. It is somewhat later, in the social evolution of men that priesthoods arise and take control over the sacrificial rituals of men and transform it into an actual tax for their benefit.
Read the second version of the 10 commandments in Exodus 34 where emphasis is put on sacrifice compared to Exodus 20, first version of the 10 commandments.