Let me start by saying that I defend anyone's right to think whatever they want to, unless they're telekinetic, in which case sensible limits are in order.  Belief is really just a stronger word for a thought or opinion, so believe what you want.  It's what you do that matters to me, i.e. don't blow me up, or elect a bunch of warmongers because you think they'll put a stop to gay marriage (they won't, and don't even care to), or ring my doorbell Sunday morning and tell me not to get a transfusion if I need one.  

    I also want to make it clear that, for the most part, I don't really agree with Atheists.  An Atheist believes there is no god, which is about as useful as believing there is one.  Both sides insist that they know something that is currently unknowable.  That's why it's called a belief, because you don't know. 

    Agnostics?  Lazy!  How do you know that you can't know something if you never try?  For that matter, how can you know that you can't know?  Isn't that knowing?  You make me sick!

    That said, here is the answer.  Religion is an evolutionary construct, period.  The trait that is our biggest asset, evolutionarily speaking, is our ability to think, but it is also our Achilles heel, since it also means we know we're gonna die.  In evolution, certain traits emerge that can help a species survive, or lead them to extinction.  Each evolutionary crossroads is like one of those "create your own story" books from the '70s.  Man discovers tools, fire, etc, and those that excel with these things survive, and those who do not excel can only survive by not needing those things, or by mastering/serving those who do.   Pretty soon, we develop new organizing principles of different kinds, like nomadic tribes, or villages, or whatever.  Our motivations are pretty simple, eat, don't get eaten, reproduce.  As we evolve, there need to be checks and balances on different types of people in order to create a stable society.  The brutish dummies need to not kill the smartass wimps, or there'll be no wheelbarrows or crossbows.  Conversely, the smartasses don't need to be so brutish because the brutes need them around for new inventions and to give wedgies to.  Still, the instinct toward self interest always threatens this balance, and many tribes get conquered or die of rampant virginity, until some genius figures out that most people are not able to internalize the concept of the greater good on a regular basis.  There needs to be a way to motivate people to do things for the greater good, while fooling them into thinking it will benefit themselves more.  Enter religion!

    Now, religion will have already been there in the form of dumbass caveman science, like "rain is the dinosaurs crying."  People can't stand not to know something, which is what makes some of us great, but most of us will just make some shit up.  So, pretty soon, there are pretty elaborate stories to explain things like fire, the stars, and farting.  These are tailor made for conditioning and controlling the masses of dumbasses, and it offers an escape from the knowledge of our own mortality.

    Knowing you're going to die sucks, especially if you're a primitive dumbass, and it can lead people in a number of dangerous directions.  The middle 50% will just eat, sleep, and screw no matter what.  The bottom 25% will see that there are no real consequences to their actions and will act on pure, brutish self interest.  The top 25% (liberal bias) will get all Emo and not want to do shit.  Hence, the wiseasses don't make new hut designs, and the dumbasses hunt and keep all of the food for themselves, and humanity becomes extinct.  Unless...what if... you have to share, and not kill, or steal, and if you do, you'll be punished after you die!!  And if you're good, you get to live forever after you die, in eternal perfection!!  Well, who the hell's gonna believe that?  Oh, sorry, Og, I didn't see you there praying to Gakos, the high lord of oddly mummified lemurs.  Gonna make for a good hunt, you say?

    See, the reason it works is that smart people believe it, too.  Sure, some of them cloak it in intellectual crap like "Gaia Theory" or what have you, but it's the same shit, because we have to believe that we're special, and that we're not "just going to die."  This is perfectly understandable, and is just fine as long as it serves it's purpose.

    The problem arises, however, when those who control the construct (religion) begin to succumb to self-interest, and twist it to their own ends.  This is as inevitable as it is suckish.  Then, you get, "Well, I know God or Gakos or whoever said we shouldn't kill, but he also just told me that we are his chosen people, so we get a pass on killing those other people.  Oh, and I get to bang your daughter. "

    Also, as people evolve, the construct can evolve, too, and the aims can be loftier, the rewards/penalties, more esoteric.  Witness the difference between the Old and New Testaments, or the progressions in Greek mythology.  (I have an ancient Greek buddy, and he gets really offended when I call it "Mythology").&nbs p; At first, it's like, "Do what I say or die!", but later, it's toned down, like "Be kind and you shall be rewarded in your spirit."

    If you look at the great civilizations that have died out over the eons, you'll see that they always flamed out just when they reached their apex of reason and were about to throw off the yoke of religion.  That is because there needs to be a limit on reason, or society can't progress.  Reason doesn't drive one to conquer, or invent, or to create. 

    So, there you have it.  That's why we have religion, and why we need it.  It's also why we have science, but that's another story.