Most people that believe in God (or a god) do so because:
1:They want to feel 'holier than thou'.
2:Insurance Policy (In case afterlife really exists)
3:Find meaning for their life
Of course, I didn't include power in here, because the Catholic Church has lost any ability to manipulate honest believers with their lies in the present world.
I believe in God not because I was raised to do so (though I was, it didn't really affect my decision), but because I know what I read in the Bible is the truth. I know I sound like some stupid jerk that would believe in sasquatch if they saw a fuzzy video of someone walking around in a monkey suit, but that's not the case. My outlook on everything is false until proven true. However, I find that the Bible contains meaning which you cannot find in anything else. It strikes deep to my outlook on life; and the words I read have an uncanny habit of relating and resolving the current problems I am facing. The Bible is better than any psychologist or best friend's advice.
This is why I get very disgusted with people claiming to be Christians decide to pick and choose what parts of the Bible they want to read. The United Church says there is nothing wrong with homosexuality, because everyone else says it's OK and they don't want to look different, as well as the US Government printing "In God We Trust" on their coins, and allowing gay marriage. Televangelists openly sell the word of God and pretend to heal people with invisible illnesses. Some pretend-Christians say they believe in human evolution from apes when there are verses in Genesis clearly against this. As is said in the Bible, don't water down the Word of God with other beliefs, as you wouldn't drip poison into a fresh spring. So clearly anyone trying to find common ground with a non-believer cares more about what others think than God, and has no faith.
The point I'm getting to is that about 70% of the people that say they believe in God do it for show, and thus their reasons for doing so will be insubstantial and meaningless.