A fish has a mind with which it assesses its immediate situation. The fish is most probably not reasoning, but the basic apparatus for perceiving the world and deciding own actions based upon the outer and inner (e.g. hunger, fear) situation is present.
From an evolutionary perspective, a single celled organism would have come somewhen before the emergence of the fish, the human being would have come somewhen after it. To make it short (now don't clobber me): Single cell = stupid, fish = aware, human = reasoning.
The ability to summarize perception in one central system which then decides for the whole being has risen with the emergence of higher life forms.
The amount of data and the size of the decision system has become more and more complex.
A fish can believe that there is a predator in its immediate vicinity and hide accordingly. But maybe the fish was startled by something not remotely dangerous.
A human can believe that there is a creator in its immediate reality and behave accordingly. But maybe the human's decision system made the fish-mistake, only on a much more complex level.
The human brain is so powerful that it can virtually create organs within itself. I am talking about association structures here. If a mind pattern becomes complex/big enough, it is autoapplied, sometimes even against the benefit or even against the will of the owner of the mind. Example: Catchy tune.
The evolutionary function of belief: There is none.
Evolution tries all possible ways (thus on its way also building tons of walls, for example a predator species that subtracts the possibility for sheep to survive in that immediate location), nature kind of stuffs shapes of life into its 3D playground as much as possible.
Also, there is technically nothing like "knowledge" - *everything* is belief. Or how do you know (=believe without the possibility to be wrong) that you are not imagining the world around you and this text? Maybe you are God, dreaming, and this text is just a mind-effect in your "head", just a process of thinking. Maybe you are the universe and every human around you.
Of course you don't believe to be alone, neither do I, but this is basically purely emotional reasoning. You don't want to die or suffer, hence you stick to reason / things that worked out in the past, kept you safe etc.. There is of course also the positive emotion-argument: It just feels good to be together, not alone. The opposite of fear of death/suffering would be the joy to be alive, the strive for bliss.
Belief is just a game of nature. As long as it's viable, why not. If it were a problem, it would be removed. Mr Dillahunty, a tool of nature, is already on it.