What is the use of the hymen?

During the early stages of fetal development there is no opening into the vagina at all. The thin layer of tissue that conceals the vagina at this time usually divides incompletely prior to birth, forming the hymen. The size and shape of this opening (or openings) varies greatly from person to person.

Sometimes this formation of an opening does not occur, resulting in an imperforated hymen (it lacks the more common opening). Some females have no hymen at birth at all, since the tissue divided completely while they were still in the womb. (source)

And, from Wikipedia:

The hymen has no known anatomical function.

In late 2005 Monica Christiansson, former maternity ward nurse and Carola Eriksson, a PhD student at Umeå University announced that according to studies of medical literature and practical experience, the hymen should be considered a social and cultural myth, based on deeply rooted stereotypes of womens' roles in sexual relations with men. Christiansson and Eriksson support their claims by pointing out that there are no accurate medical descriptions of what a hymen actually consists of. Statistics presented by the two show that fewer than 30% of all women who have gone through puberty and have consensual intercourse bleed the first time. Christiansson has expressed an opinion that the use of the term "hymen" should be discontinued and that it should be considered an integral part of the vaginal opening.

The slang term "cherry" is also commonly used as a reference to a woman's hymen. Specifically, the phrase "popping the/her cherry" indicates a loss of a woman's virginity. The cherry may have been chosen because its bright red color and juice is similar to blood, and the fact that some women bleed when having intercourse for the first time.