A method of control, a way of avoiding responsibility, a method of comfort for those that are unwilling or don't understand how to accept death for what it is, and a flawed concept around which some social networks form.
Aside from the obvious political reasons and a way out of responsibility to think for yourself, consider this (based on Joseph Campbell's ideas about the origin of god.)
Imagine hunting a wooly mammoth. It is awesomely massive. It is fearsome. It is, most definitely, more powerful than the strongest in your tribe. But it is winter and that coat would clothe many, that meat would feed many for a long time, and those tusks would make some great tools.
So, the wily humans cooperate and hunt one down - using spears and some form of rope and a strategy. The battle is a very heated 'in the now' animal moment. The men are afraid to the point of being high on adrenalin, dopamine, endorphins, etc. (the most powerful drugs known, BTW). But, in all this, they notice that the beast is also afraid. It fights for its life. It has calves and a herd. It kills and it dies.
So, the tribe is aware of many things: an awesome creature who didn't want to die any more than they did and fought as bravely for its life as any one of them; has provided, with its very life and being, warmth, food and tools for the tribe. Some would, certainly, respect and revere this great creature - to a level that might reach a kind of deification of it. This extrapolates to a deification of nature itself. And, where the survival of the race is concerned, this is probably a useful notion.
But, when the tribes get big enough, when civilization begins to shape up, when people begin to control their environment - religion becomes more of a political tool than a reverence for nature. Indeed, the reverence for their political gods becomes counter to their reverence for life itself. Certainly, it destroys any reverence for using their own minds, their own experience, and their own agency.
From an article in The Onion about the most important inventions of the last 4.5 billion years..."God: This multipurpose tool has allowed billions to soothe their mortal fears while easily excusing a wide variety of unconscionable actions such as war and homophobia."