The proof for God's existence always seems to be "why?"... and I always retaliate with "why does God exist?"... It's a valid argument, but it has gotten a bit tiresome. So here's a new one: To answer your question "why are we here", because God made us. Now why did omniscient, omnipotent, omnipresent God create lowly humans? He just got bored one day and said "Hey, I know! I'll make little people who will kill each other, question my existence, rape, give me 10% of their income (because I may run low on my cash supply), and so much more! Then I will rest on a Sunday (because I will get tired after all the work done by my infinite power), and then I will become so emotionally attached to them that when I send them to hell, I will feel sorry for them, but hey! I made hell too, so it must also be perfect... And rather than inspire hope, and love, I will inspire fear! This way when they get mad at someone, they can say "I guess I should just forgive and forget... WAIT A MINUTE!!! Why, when I am created in God's image, should I forgive and forget?!? If I did something that God did not like, he would send me to hell for all of eternity!... Unless of course I politely ask for forgiveness. And then I get to eat the greatest most perfect fruit ever for the rest of my existence... but of course I could already be doing that if that bitch Eve hadn't went and screwed over man-kind by eating an apple." -- I could go on for hours with this BS, but I think I may be digressing a bit.
Through all this, what I am trying to say is: why did God do any of this? Also, God himself belongs in hell. If I tortured someone for all of eternity because I personally didn't like their actions, I surely would be sent to hell.
I apologize for only considering my idea of God in this, but I'm sure that a good portion of my thoughts would apply to most other 'Gods' too.