Photo: Crocheted Penis Pillow, courtesy of Motives and Patterns, at Etsy.
Evolutionary biologists believe that the shape of the penis has an important adaptive function. They suggest that the corona acts as a plunger and scraper, removing sperm deposited by a previous reproductive partner.
An alternative, but somewhat similar, explanation has been proposed. Via Discover, and published in Archives of Sexual Behavior:
It has been proposed that the shape of the glans with its distinctive corona facilitates the scooping out of previously deposited semen, enabling the subsequent deposition of other genetic material. This is not a likely occurrence. The healthy vagina has a low pH to protect it from invading organisms. This intense acidity will kill all sperm not promptly reaching the safety of the cervical mucus. I suggest the evolutionary function of the glans and its corona is not to scoop out semen but vaginal secretions, which are then absorbed by the encompassing foreskin. The mucous membrane of the foreskin adjacent to the glans is highly absorbent, as evidenced by the increased susceptibility of the uncircumcised male to HIV. It has been suggested that there may be a glans protecting feature of the foreskin, but it is entirely possible that its primary function is to absorb behavior modifying chemical messengers from the female. Numerous studies have shown the vaginal mucosa to be secretory. In the primate rear-approach coital position, the corona rubs against the anatomically posterior wall of the vagina, the most dependent surface when the woman is standing, and therefore the place more likely to collect vaginal secretions. The deepest end of the posterior vagina is the cul de sac, which commonly contains a small amount of fluid. An analytical study of the vaginal secretions of an ovulating female for neuropeptides, such as pitocin and vasopressin, known to enhance pair bonding and protective behavior in the male, could be revealing. Sexual intercourse may be a medium of communication between humans older than language itself.
Full reference for the paper:
Edwin A. Bowman, E. A. (2010). An explanation for the shape of the human penis. Archives of Sexual Behavior, 39, 216.