Throughout the history of mankind, humans have followed an astonishing variety of belief systems. Different societies have worshipped everything from the sun and the earth to any number of gods from one to hundreds, covering everything in between including cats! These fascinating belief systems are dependant on geographical location and period of history - thus can be seen as having strong cultural roots. Logically, no more than one of the thousands of religions can be correct - either there is a god or there isn't, there is reincarnation or there isn't. It seems arrogant to the point of incredulity to believe that any of the current ideas are anything more than the latest incarnation of this evolving, organic process. It is important to note that the current fad in the western world for "spirituality rather than religion" is exactly the same phenomenon. This cannot be differentiated from paganism, christianity, islam, buddhism or any other belief.
So, why? Why do humans need to believe in anything? Because it is a fundamental part of being human to put our trust in an externality rather than accepting responsibility for our own life. How much easier is it to trust the outcome of an exam or job interview to fate or the hands of the gods or karma than to our own efforts. A sacrifice of a goat to ensure a good harvest fulfilled a human need at a time when knowledge did not extend to an understanding of the climate or agriculture. In the same way now, prayer is a useful retreat in times of hardship and worry (unfortunately the predominant state for most people most of the time...).
This is not to say that religion is "a waste of time". That statement is no more insightful than saying fear or worry is a waste of time. Indeed religion has provided invaluable comfort for billions or people throughout history to help get through difficult times. It is perhaps simply unfortunate that prayer has no more of an effect on the world than sacrificing that poor goat. It is our actions that make a difference. And the crucial first step in living a happy, worthwhile life is to accept full, absolute and unconditional responsibility for everything that happens in our lives. Religion can't help in the same way that worry and fear do not help. Get rid of them all.