On all (or at least the vast majority of) baseball caps, there is a small button on the very top, which has led to speculations regarding the actual purpose of it. Some say it's functional, to keep the panels together. Other say it's just a matter of design. But what's actually the main purpose of it?
Being located on top of the cap, where all the panels meet, the top button might be regarded as a functional detail, keeping the cap crown in one piece. Some say that historically, the button actually had that role. Though, in modern time, the button works as a cosmetic feature. The use of the button is to simply cover the joint point where the panels of the hat meet - often with the same or matching fabric to the cap crown. Some caps do not have a button, but one can argue that a cap actually looks better when the seams are covered.
Fun fact: The top button's nicknames are "Squatcho" and "Squatchee".