I have no hair so I come from the point of view of sun protection, but I can say that a cap makes the person feel secure and complete, because your head feels comfortable when it is contained within something. The cap is an American styled hat made popular during the mid-century at baseball games, so for the citizens of the USA it carries a 'ballgame' feel, ie: relaxing with family on the weekend. Generally people have tried to introduce this comfort/relaxing mode into the other areas of their lives, such as work, exercising, filling the car. As the cap became more popular in the lives of Americans it flowed into popular culture through TV and movies, thereby spreading its' presence to other cultures, in NZ where I live we recognize that the cap is a USA import but we have stamped our own island style on it so that it is part of our own culture. When all is said and done there aren't many other trappings that a man can wear with street credibility, since high fashion and handbags have traditionally been in the female domain.