Swimming caps show up at professional swim meets, triathlons and at pools where practice laps take place. Latex and silicone are two common materials for competitive swim caps, with both offering similar advantages in the water. While swim caps once were used primarily to protect the hair style of the wearer, they now serve more functional purposes for swimmers.
Protection
The swim cap fits snugly over the head, covering the hair and scalp. The covering protects those areas from the chemicals in the pool. Chlorine in particular damages the hair and may cause discoloration. This is especially a problem for those who spend lots of time in the pool practicing for competitions. A swim cap can also protect the head from cold water, particularly in an open-water swimming situation when the water is very cold. The cap helps hold in your body heat and keeps the water from directly touching your head. When swimming outdoors, the cap helps protect your head from direct sunlight.
Reduced Resistance
Your hair creates drag that can slow you down in the water. The snug fit of the swim cap keeps your hair pressed down and covered to reduce the amount of resistance as you move through the water. The reduced resistance is particularly useful for those who swim competitively. It also allows competitive swimmers to keep their hair longer without the worry of the hairstyle slowing them down in the pool.
Identification
A bright swimming cap helps identify a swimmer in the water when it is otherwise difficult to differentiate between multiple swimmers. In some cases, the swimmer's name or another identifying mark may be imprinted on the cap. In a triathlon, a brightly colored cap makes it easier to see other swimmers when there is a large group of people swimming in one area. This helps swimmers avoid bumping into one another. The race coordinators may use different colored caps or require you to wear one in a specific color that they provide in order to help with identification.
Cleanliness
Keeping your hair securely under a swim cap means you won't shed any hair during your swim. This can help keep a pool clean. When hair falls out in a pool, it often ends up in the pool's filters. The hair may clog the filters or make them dirtier. The pool's operator may request that swimmers wear caps to prevent this buildup in the filter system. The caps also reduce hairspray and other hairstyling products that otherwise end up in the pool if the swimmer doesn't wash her hair before swimming.