The purpose of the menstrual cycle is to
expel an unfertilized egg and endometrium
bring nutrients to the egg
indicate that the egg has been fertilized

The importance of the menstrual cycle is to expel an unfertilized egg.
The menstrual cycle is termed as a natural change that occurs in the reproductive system of a female. To be specific in ovaries and uterus which makes pregnancy possible. This cycle is important for the production of oocytes of prepare uterus to hold a pregnancy, and it is being governed by hormonal changes. The menstrual cycle consists of three phases which are the follicular phase, ovulation, and luteal phase. 

Fossil found in strata reveal that unused body part decrease in size