What purpose does religion serve?

What good comes from religion and faith in general or Monotheism in particular?

Good question!(Though it's really two questions.)

Religion has some practical benefits, though none that are not as important now as they were in the past.

Human societies consisted of relatively small bands of hunter-gatherers for 99.9% of "human" history. In such small bands you would know EVERYONE. You could quickly learn who you can trust and who you can't trust. (Those who were untrustworthy were often killed or expelled.)

With the invention of agriculture, societies got so large that individuals couldn't still know everyone. So it was difficult to know who to trust.

Organized religion guaranteed that people at least had the same moral influences (if not completely identical actual values). So a certain level of trust could be established between individuals in such larger societies. Some level of trust is needed in societies with large populations to even allow them to exist without lots of internal conflict. Having a central church, often controlled by the political leader in the society, also improved the ability of a centralized ruler to govern.

Also my personal belief is that at some point humans started to find explanations for things that occurred in nature.

For example, if you plant a seed a plant will grow.

But once humans found SOME explanations they became uncomfortable when things occurred that had no apparent explanation. It created uncertainty and humans have never liked uncertainty. A God (or gods) provided an "explanation" (so to speak) for those things that occurred with no apparent explanation.Rain and rain gods are an example.

Additionally, certainly some people find inspiration in a belief in a higher power. For reasons I've already mentioned, we probably evolved that need or ability. But however it came about, no one can dispute that it exists.

Those individuals, at least, would find life a bit empty without a belief in God.

Monotheism - your second question - probably came about because it allows power to be concentrated more effectively when there is only one God to worship. Sometimes polytheists have different priests who specialize in each of the many different "gods". There is then a power struggle to determine which god, and therefore which priest, is the most important.

I personally have no problem with religion or religious people.

I DO have a big problem with fundamentalism.

I define "fundamentalism" as the belief that a human being can have an inerrant and infallible understanding of God (if God exists). Generally fundamentalists think that such an infallible understanding is achievable through some holy book or some specially ordained prophet.People who think that they are getting orders directly from God can allow their beliefs to bypass common sense on their way to becoming real actions.

Fundamentalism has done MUCH more harm than good in human societies over human history.