In my copy of the book, the best place to find this answer is on p. 278. There is a paragraph right before the section break on that page that explains it. Diamond says that there are two reasons why religion is useful.
- It helps to allow people to live together. Why should people who aren't related live together without killing each other or stealing from each other? Religion is one way to give people a reason to live well together.
- It gives people a motive to fight and die for their society. If they do not a reason, they will not fight to defend their society. Religion is one way to provide a reason -- fight for your god and your country.
Because “kleptocracy” is not a term used much outside of Guns, Germs, and Steel, and because that book discusses the ways in which religion supports kleptocracy, I assume that your question is from Diamond’s book. I have edited and moved it accordingly. If my assumption is correct, the answer to your question can be found on p. 278 in the book.
On p. 278, Diamond says that there are two ways that religion can be of use to society, other than simply helping to maintain the status quo of the kleptocracy. First, religion helps allow people who are not related to one another to live together in peace. At the beginning of Chapter 14, Diamond talks about how hard it is for unrelated people (and people who have no personal ties) to live together without violence. Religion reduces this problem. It gives people something that binds them together (they both have the same religion) and therefore makes violence less likely. Second, Diamond says, religion gives people a reason to sacrifice themselves for the community as a whole. They will be more willing to die for their community in war if they believe that God (or the gods) supports that community. If religion makes people more willing to die for their community, the community will be more powerful in military terms.
These are the two reasons that Diamond gives for why religion serves society other than by simply supporting the kleptocracy.
The real answer is on the last paragraph of 277