pubic hair

Pubic hair actually serves several purposes:

• It's primary purpose is to trap pheremones. This emits an odour which sexually attracts and increases arousal in potential mates.
  
• It signals that a person has entered sexual maturityand is a valid partner.
  
• In men it can trap air and therefore help regulate the temperature of the testicles, which is obviously important for men.
  
• They're a healthy source of protein. Or at least I hope that they are. There was one in my sandwich today.

The American Board of Dermatology actually had research as to what makes it curly and why:

Pubic hair is curly because it is genetically made in a flat shape rather than a round shape. Perfectly round hair, such as those on scalps of Native Americans, is perfectly straight and has no tendency to curl. However, ribbonlike hair on the scalps of blacks is seen to curl because of its oval-like construction. The same is true of pubic hair... --Dr. Joseph P. Bark

But what function does the curliness serve? Well, some dermatologists speculate that if it grew straight and stiff, it would rub against adjacent areas and cause discomfort. It also grows much slower, so you don't trip on it I guess.

It's not always curly. Early in puberty it's soft and straight. Most Asians have sparser and straighter pubic hair than that of whites or blacks.

--quote taken from Do Penguins have Knees? by David Feldman

During my research on the topic of why we have pubic hair, I found that most sources agree: there is no definitive reason why we have pubic hair.

The most plausible and widely accepted reason why humans have pubic hair is related to scent. Apocrine glands release pheromones, which get trapped in the pubic hair. The combination of these pheromones and decomposing bacteria that gets caught in the hair, combine to define your scent and stimulate the libido of possible partners.

Here's a few other possible reasons for pubic hair (not all of which make good sense or are generally accepted):

Some theories suggest that pubic hair is to provide warmth and reduce temperature fluctuations around the genitals. But others discount this theory, indicating that the amount of hair is insufficient to make any significant difference.

Pubic hair (and underarm hair) also functions as lubrication. Pubic hair in particular protects against the friction generated during intercourse. Under the arms and between the legs, skin on skin contact causes chafing. But the natural oils from hair and the movement of the hair itself prevents direct skin on skin contact.

Pubic hair also indicates a level of sexual maturity to other humans. The growth of pubic hair usually coincides with the arrival of the stage of puberty where reproduction becomes possible.

Cecil Adams from straightdope.com reports that a Smithsonian monograph "presents the novel thesis that pubic and axillary (armpit) hair gives babies something to grab onto". Ouch.

And finally, research indicates that humans have just as many hair follicles over their bodies as apes do; the majority of human hair is simply finer and more lightly coloured. It's been said that human body hair is just leftover from the process of evolution, when primitive humans were covered in hair to provide warmth.


Sources
Ask Alice
Wikipedia.com
straightdope.com