Well, for one thing, it's your body's way of preparing for pregnancy. Once you get your period, it means that you are physically able to become pregnant and have a baby – although you don't necessarily feel ready!
Periods happen as a result of hormones released from a gland at the base of your brain called the pituitary gland. These hormones cause an egg to be released. At the same time, the lining of your uterus (or womb) is becoming thicker – like a soft and spongy bed, ready for the egg.