The physiological purpose of the hymen is one of the eternal mysteries of women's bodies. Although it doesn't seem to have a specific function, it's thought that hymen tissue remains as a vestige of vaginal development. Embryologically, it tended to keep germs and dirt out of the vagina.
Hymens vary in shape, size and thickness
Medically speaking, however, the concept of an "intact" hymen is a myth. That's because hymens vary in shape, size, and thickness. Among the multiple possibilities are hymens that surround the entire vaginal entrance, with an open space in the center (called an annular hymen), and hymens that appear open with a thin line of skin down the middle (a septate hymen).