You can create whatever artwork you want on them. Most of the sites will show you what to do and give you a preview of the finished product before you purchase. This is convenient for when you need to make hats for events or a team event.
Some people like to collect fitted caps. There are old caps that are no longer made that make an interesting collectable. Collecting fitted caps can be an expensive hobby sometimes. Some caps go for almost 40 dollars a hat, but most will run about 10-15 dollars. It is an enjoyable hobby and you can show of your style.
Either for getting a shade, for warmth, or for decoration, wearing hats has become a current trend. Each individual may has personalized taste. You may experience 'shopping around' until your ideals come into sight. Different hats are worn in various season, difficulty is what's good during the period of changing seasons. Hats are now considered main apparel in the modern world.