If a person believes in a diety, they do so because that diety is a part of their fundamental beliefs.
A person might adopt a religion that includes a diety for many reasons, but most derive from the same fundamental situation: the human situation. By this, I basically refer to man's need for reassurance in the face of an inexplicable environment.
This environment entails not just physical phenomena (lightning, thunder, death, birth, the physical origin of the universe) but also purpose
. The eternal why?
A diety helps explain both the physical and spiritual quandries of mankind if placed in the right context. So does science.
I'm hoping that you know all of this. What I'm also hoping is that eventually you'll realize that the kind of "investigation" you're conduction serves absolutely no purpose. You'll learn nothing by running your mouth. If you truly, really want to understand another person's point of view, you need to empathize with that person - to understand the basic circumstances that helped formulate their beliefs and to understand how their beliefs address those circumstances. For instance (a very brief, non-comprehensive for instance) Judaism helped provide a unifying force for a numerically inferior and landless people. You'll also need to investigate the spiritual needs of a people. If you listen to any of the angst-punk-pop on the radio, you're more than capable of this.
In short, I'm saying what the first person to respond said. If you really want to know, shut the fuck up and research, observe, listen to people. If you don't really want to know, then just shut the fuck up.