Understanding the Monthly Menstrual Cycle: Why Does It Happen?

Question
I was wondering if you ever read the reason 'why' women have a menstral (not sure how to spell it, sorry) cycle. I mean, the body is 'throwing-away' blood and tissue. When we get cut, our body works over-time to stop the loss of blood. But with the monthly 'period' the body willingly tosses away blood and tissue. I am not sure why this would be normal and monthly.

Thank you for your time.  

Answer
I think it's normal because the body is getting ready each month to make a baby.  When it doesn't happen, the uterine lining has to start over getting ready to make a baby.  There might be other ways for the uterine lining to prepare for a pregnancy, but this seems to be a good one.