3 Answers

If you ever watch boxing, MMA, or any other physical sport. You will see sometimes people will get knocked out from a hard hit to the head or jaw.

Basically it's when your head gets rattled so hard that your brain hits your skull and that's when you faint. The reason your body does this is because when you're standing gravity pulls your blood down. When you faint, your body goes limp and the blood gets to your head more easily when you're laying down.

http://explainlikeakid.blogspot.co.uk/2011/09/why-you-faint-when-you-get-hit-hard.html

Read this:  http://topics.info.com/When-to-Call-a-Doctor-After-Passing-Out_4980

Yes as Romos has stated it is a strategy of biology, it is an adaptation to help to preserve life, very clever of Mother Nature, I would say!  Also, while you are out, you do not feel the pain, temporary, as you come to, there it is again...