We hear these questions come up often. Eyelets are the small holes, about 1/8 in diameter, that are punched in the middle of each fabric panel in a baseball cap (most baseball caps have 6 or 5 panels). Eyelets are most commonly found in cotton, wool, or polyester fabric panels, and are usually not found in mesh fabric panels. The reason for this is that hat eyelets are constructed for ventilation purposes. For non-breathable fabric panels, eyelets make the hats more comfortable on hot days and reduce sweating. Eyelets are not needed for mesh panels since mesh is already breathable.
Eyelets on a cotton baseball hat - top, middle of each fabric panel
Eyelets have also become a style feature. For example, some baseball hats offer contrasting eyelet colors designed to stand out from the main fabric color, even on mesh caps. Contrasting eyelet colors are often combined with contrasting sandwich visors and buttons on the top of the cap.
Contrasting color eyelets on a cotton baseball hat for style purposes