Once upon a time, wearing a baseball cap meant something. First, it meant that you played baseball. Soon, it was for fans of baseball or people who needed the sun kept out of their eyes when they watched a baseball game. Before long, baseball caps became a standard part of many men's wardrobes, and it is rapidly becoming much the same for women as well.
Promotional baseball caps are an excellent spirit item because people notice them instantly. It's very difficult not to automatically check to see who or what is represented on someone's baseball cap. It's a reflex that never goes away, and it's stronger the more vibrant or colorful a cap is. So many people end up wearing the same cap for the same team in different cities, so anything outside of the norm is automatically intriguing.
There are also quite a few people, particularly in college, who amass large collections of baseball caps. These folks are walking cap billboards, and every day is a new installment. People who know these serial cap-wearers well are always curious to see which cap they'll be wearing on a given day. As a result, having your logo or brand in such a person's collection is like being endorsed by a promotional products celebrity.