Some public uses of religion (positive and negative):
Religion does make strong bonds between people — stronger than secular ones do because you have the force of an external God (imaginary or not) to bind you. It can give people a Universal, God-given reason to do certain more extreme acts that a person on his own would not do, naturally — some good, some evil — but for the good of his group to survive long enough to produce offspring, that survive long enough to have offspring themselves — an evolutionary advantage. The extreme acts of the Muslim Martyrs, Christian Martyrs, “God on their side” soldiers are examples.
Supposedly, there are also things in cultural evolution called memes (instead of genes) that are elements of culture that survive according to cultural natural selection. e.g. the trinity concept. So, in people, various supernatural memes may predominate for a while (sometimes a long time) and eventually change, are displaced, or disappear from human society. These memes do not have to reflect reality — they can be totally imaginative and fanciful. Memes often change and evolve much, much faster than genes do (in less than a generation as opposed to many years for gene effects to accumulate).
(1) In times of war, religion can and is used to help people think of people on the opposing side to be monsters that must be exterminated. Thinking of people as being the enemy of God is a much more effective motivator to kill than, secularly: to think of people as just people on the other side. It may be necessary to emphasize supernatural religious beliefs to motivate the populace to make sacrifices that are needed for survival, especially if the opposition is doing the same on their side.
(2) Similar to item (1) above, except applied to certain categories in society — not to kill, but to rationalize limiting their freedoms.
(3) In a natural or man-made crises public religious influence can motivate people to help and contribute to those in need. Of course, secular organizations can do the same without any religious strings attached.
Naturalism and Religious Experience 