Why is it that religion alone has the rites called yajnas or sacrifices?
If a crop grows in surplus in our place we trade it with what is available in plenty in another and is not produced in our own. The carpenter, the blacksmith and other artisans make useful articles and serve us in many ways. In return we give them what they need for their upkeep. We feed the cow grass and it yields us milk. We pay the government taxes and it gives us protection. The affairs of the world are conducted on the basis of a system of exchange. Similarly, we conduct an exchange even with worlds other than our own. Engineers and other experts can canalise water obtained from the rains but they cannot produce the rains. If we want the rains to come, we have to despatch certain goods to the abode of the celestials. It is this kind of exchange that the Gita speaks of:
Devan bhavayatanena te deva bhavayantu vah
Parasparam bhavayantah sreyah param avapsyatha
It means: " You keep the devas satisfied with the performance of sacrifices. And let them look after your welfare by producing rain on earth. Thus, helping each other, be more and more prosperous and happy. "
For a general background, please see here